The ornamental design for a portion of a toothbrush, as shown and described.
FIG. 1 is a top, front, left perspective view of a portion of a toothbrush showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The uneven-length broken lines represent the bound of the claim and form no part thereof. The even-length broken lines showing the remainder of the portion of a toothbrush are for environmental purposes only and form no part of the claimed design.