The ornamental design for a seat for an exercisedevice, as shown and described.
FIG. 1 is a front elevational view of a seat for an exercise device showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a front perspective view thereof; and,
FIG. 7 is a perspective view thereof on an exercise device.
The bottom plan view is not visible during normal use.
Those portions of the exercise device shown in broken lines are included to denote nature and environmental use of the claimed design and form no part of the claimed design.