The ornamental design for belt clip for a firearm, as shown and described.
FIG. 1 is a top, front, right perspective view of a stock for a firearm showing the new design.
FIG. 2 is a right side view thereof.
FIG. 3 is a top view thereof.
FIG. 4 is a front view thereof.
FIG. 5 is a left side view thereof.
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a back view thereof.
The broken lines located inside the solid line disclosure represent unclaimed features not part of the claimed design. The broken lines located adjacent the solid line disclosure and showing a firearm are for illustrative purposes only and form no part of the claimed design.