The ornamental design for a portion of a sandal, as shown and described.
FIG. 1 is a perspective view of a portion of a sandal, showing my design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of the heel, base and bottom of the shoe, connecting hardware, and pattern on the interchangeable top is included for the purpose of illustrating environmental structure and forms no part of the claimed design.