The ornamental design for a portion of a whisk, as shown and described.
FIG. 1 is a perspective view of a portion of a whisk showing my new design;
FIG. 2 is a side view thereof;
FIG. 3 is another side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a front view thereof.
The broken lines immediately adjacent to the shaded region form unclaimed boundaries of the design. The broken line showing of the remainder of the whisk is for illustrative purposes and forms no part of the claimed design.