We claim the ornamental design for a portion of a seat, as shown and described.
FIG. 1 is a perspective view of the ornamental design of a portion of a
seat; and,
FIG. 2 is a front view of the portion of a seat.
The broken line showing of the environment is for illustrative purposes
only and forms no part of the claimed design. The broken lines which
define the stitching pattern of the seat form no part of the claimed
design. The top, bottom, and both side views are not part of the claimed
design.