The ornamental design for a clock, as shown and described.
FIG. 1 is a Front Perspective view of a clock, showing our new design;
FIG. 2 is a Front Elevational view thereof,
FIG. 3 is a Rear Elevational view thereof,
FIG. 4 is a Right Side Elevational view thereof,
FIG. 5 is a Left Side Elevational view thereof,
FIG. 6 is a Top Plan view thereof; and,
FIG. 7 is a Bottom Plan view thereof.
The structural features depicted by broken lines in the drawings have been shown for the purpose of illustrating portions of the clock that form no part of the claimed design.