The ornamental design for a grip for a writing implement, as shown and described.
FIG. 1 is a perspective view of a grip for a writing implement showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines in the drawings of FIGS. 1-6 are included for the purpose of showing portions of the grip for a writing implement that form no part of the claimed design.