The ornamental design for a clamp, as shown and described.
FIG. 1 is an oblique view of the clamp of the subject invention, with the clamp being attached to a breadboard by a threaded bolt, and with the clamp fastening a pedestal to the breadboard; the breadboard, bolt and pedestal shown in broken lines depict use and environment only and form no part of the claimed design;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is an oblique view thereof, shown without environmental features;
FIG. 5 is a right side elevation view thereof, the left side being a mirror image of the right side shown;