The ornamental design for a portion of a wiper, as shown and described.
FIG. 1 is a front right perspective view of a portion of a wiper showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top view thereof.
The broken lines immediately adjacent to the shaded areas represent unclaimed boundaries of the claimed design. The remaining broken lines are for illustrative purposes only and form no part of the claimed design.