We claim the ornamental design for a clamp, as shown and described.
FIG. 1 is a first perspective view of a clamp showing our new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines adjacent shaded areas depict boundaries of the claim and form no part of the claimed design. All other broken lines illustrate portions of the clamp that form no part of the claimed design.