The ornamental design for a portion of a toothbrush, as shown and described.
FIG. 1 is a perspective view of a portion of a toothbrush according to my new design;
FIG. 2 is a right side view, the left side being a mirror image thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a close up view of the portion of the toothbrush head.
FIGS. 1-7 show portions of the design in phantom lines. The portions shown in phantom lines are for environmental purposes and form no part of the claimed design. For purposes of illustration, FIG. 7 shows the design with the phantom line portions removed.