The ornamental design for an innercap for a wig, as shown and described.
FIG. 1 is a front view of an inner cap for a wig showing the claimed design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a left side view thereof.
The dashed broken lines in the figures show portions of the inner cap for a wig that form no part of the claimed design. The dot-dash broken lines in the figures show boundaries that form no part of the claimed design.