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 a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a side elevational view thereof;
FIG. 5 is an opposite side elevational view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in the figures illustrate portions of the design that form no part of the claimed design.
The shading is provided merely to represent contours of the design and does not connote the character of the surfaces such as any specific texture, material, gloss, translucence, transparency, surface treatment, or surface decoration.