The ornamental design for portion of a boot, as shown and described.
FIG. 1 is a front perspective view of a portion of a boot showing my new design
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof; and,
FIG. 5 is a top view thereof.
The broken line showing of the sole, heel, heel cap, and portion of the upper is for illustrative purposes only and forms no part of the claimed design.