The ornamental design for a clock, as shown and described.
FIG. 1 is a perspective view of a clock, showing the new design;
FIG. 2 is an exploded perspective view thereof;
FIG. 3 is a front elevational view of the clock shown in an assembled condition;
FIG. 4 is a rear elevational view of the clock shown in the assembled condition;
FIG. 5 is an exploded left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 6 is a top plan view of the clock shown in the assembled condition, the bottom, left and right side views of the assembled clock are the same; and,
FIG. 7 is a rear perspective view of the clock shown in the assembled condition.
The broken lines in the figures depict environmental matter, and form no part of the claimed design.