I claim the ornamental design for a clock, as shown and described.
FIG. 1 is a front and right side perspective view of a clock, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a rear view thereof.
The structural features depicted by broken lines in FIGS. 1, 5, 6, and 7, have been shown for the purpose of illustrating portions of the clock that form no part of the claimed design.