The ornamental design for a clock, as shown and described.
FIG. 1 is a 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 left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The clock hands depicted by broken lines in FIGS. 1 and 6 have been shown for the purpose of illustrating elements that form no part of the claimed design.