The ornamental design for a clamp, as shown and described.
FIG. 1 is a perspective view of a clamp showing my new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a third perspective view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a first side view thereof;
FIG. 7 is a second side view thereof;
FIG. 8 is a first end view thereof; and,
FIG. 9 is a second end view thereof.
The broken line showing of aspects of the clamp in FIGS. 2 and 5 are included for the purpose of illustrating environment and forms no part of the claimed design.