The ornamental design for a clock, as shown and described.
FIG. 1 is a first perspective view of a clock showing my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an enlarged view of the selected portion in FIG. 2.
The dashed broken lines shown in the drawings depict portions of the clock in which the design is embodied that form no part of the claimed design. The dash-dot-dash broken lines shown in FIGS. 2 and 9 define the boundary of the enlarged detail and form no part of the claimed design.