Is is possible to clip a drop shadow to apply only in a specific area?

Alex,

I always suggest that whenever you are about to do something destructive, such as expanding an object with a Drop Shadow, always keep a backup of the original artwork to fall back on, if you ever need to edit; an edit can be anything from changeing letters/words down to improvements of (small) details.

This is the reason for my suggesting the auxiliary Group which works without doing anything to the original artwork; if you wish to change it, you can just do so, and create a new auxiliary Group if needed.

Apart from that, it seems that you have Drop Shadows on both OverPoweR and CardGames, so you would need to apply the Clipping Mask(s) to both.

One way to use a Clipping Mask could be to:

1) Create a copy of both the OverPoweR and CardGames objects to the very front,

2) Remove the Drop Shadows from the original instances,

3) Create a copy of the triangle at the very front (and expand it if it a stroked path) and Unite it, or if applicable just create a new path with the Pen Tool following the outer shape and snapping to the corner Anchor Points (Smart Guides are your friends);

4) Select the new top triangle along with the OverPoweR and CardGames objects from 1) and Ctrl/Cmd+7 to create the Clipping Mask/Set.