Population: B i g

Method 1: Use an Invisible Material

Recommended situations to use this method: You want to quickly make the mesh underneath invisible, but only parts of that mesh (example: Hiding the visibility of Scout's torso won't work because his arms are also on that layer).

Select the part of the merc's model that's clipping through the cosmetic (in this example, it's Scout's default shirt, which is part of his torso model). In the Materials tab, select Add New Material Slot (+ button to the right of the materials list). Press the +New button to add a new material to it. (See: Image 1, below)

Rename this new material something clear (like invisible_material). Set the new material to have 0.0 Alpha (under the Surface header) and change the Blend Mode to Alpha Clip or Alpha Blend (under Settings). (Image 2)

Go into Edit Mode. Toggle select faces mode. Select the faces that are clipping. These need to be faces that never need to be seen by the camera (they're being covered by the cosmetic). (If it's difficult to select the faces, go into Local View mode with the model selected by selecting View -> Local View -> Toggle Local View, or by pressing your Local View hotkey.) Various methods to make selecting the faces easier include switching your selection mode to Lasso, and turning on X-Ray in your view (circled in blue in the attached image). (Image 3 and image 4)

With the faces selected, select invisible_material from the materials list and press the Assign button. (Image 5)

Go back to Object mode (and leave Local View if you haven't already). The faces assigned to the invisible material should now be fully transparent, leaving the cosmetic covering it.

Optional: If you wish to animate the invisible material's visibility: Go to the materials tab, select the invisible_material, and Press the dot button to the right of the shape key's Value. This adds a keyframe. Yellow means that keyframe in the timeline has a keyframe. Green means the value has keyframes but not on the current frame. Influence set to 0.0 is off, 1.0 is on 100%. This can be useful if you're animating Scout putting on the cosmetic for example.

Method 2: Use a Mask Modifier

Recommended situations to use this method: You want to make parts of the mesh underneath invisible without editing the applied materials of the mesh (example: You use the Materials list to quickly select vertex and reassigning vertex to a new material slot would slow down your process).

Select the part of the merc's model that's clipping through the cosmetic. Go to the Data properties tab and add a new vertex group (+ button to the right of the vertex group list). Rename it something clear, like Mask_mesh. (See: Image 1, below)

Go to the Modifiers property tab. Press Add Modifier. Add a Mask. In Vertex Group, select Mask_mesh. Then press the Invert button to the right of that. (Image 2)

Go back to the Data properties tab. Go into Edit Mode. Select the vertex that's clipping through the cosmetic. (If it's difficult to select the faces, go into Local View mode, switch the selection tool to Lasso, and toggle X-Ray mode--see: Method 1 above for details.) (Image 3)

With the vertex selected, select the Mask_mesh vertex group and press the Assign button. (Image 3)

Optional: If you wish to animate the invisible material's visibility: Go to the modifiers tab. In the Mask Modifier, Press the dot button to the right of Threshold. This adds a keyframe. Yellow means that keyframe in the timeline has a keyframe. Green means the value has keyframes but not on the current frame. Since the Mask is inverted, Threshold set to 0.0 is on, 1.0 is off. This can be useful if you're animating Scout putting on the cosmetic for example. (Image 4)

Method 3: Use Shape Keys

Recommended situations: There's clipping, but you want the vertex of the default outfit to stay visible (example: you want to adjust Scout's backpack strap so it's entirely layered over his new cosmetic jacket). Or you want to animate when the clipping adjustment happens (example: An animation of Scout putting on a cosmetic, wherein you see Scout in his default outfit and his his cosmetic within the same shot).

Select the part of the merc's model that's clipping through the cosmetic. Go to the Data properties tab and add a new Shape Key (the + button to the right of the Shape Kay list). This is the Basic layer (default shape of the mesh), don't ever touch it. Now add a new shape key layer (by default, it's named Key 1). Rename it something clear like Clipping_fix. Select it, and set it to 1.0. (See: Image 2, below)

Go into Edit Mode. Make sure that the Clipping_fix shape key is the one selected (otherwise, you are editing either the Basic shape key layer or a facial flex shape key, which can have destructive results). Start selecting vertex and moving them where you want. (If it's difficult to select the faces, go into Local View mode, switch the selection tool to Lasso, and toggle X-Ray mode--see: Method 1 above for details.) (Image 3)

Go back to Object mode. The shape key should now be affecting the mesh. (Image 4)

If you wish to animate the shape key: Press the dot button to the right of the shape key's Value. This adds a keyframe. Yellow means that keyframe in the timeline has a keyframe. Green means the value has keyframes but not on the current frame. Influence set to 0.0 is off, 1.0 is on 100%. (Image 4)