The ornamental design for a clamp, as shown and described.
FIG. 1 is a left front perspective view of a clamp embodying our design.
FIG. 2 is a top plan view thereof.
FIG. 3 is a bottom plan view thereof.
FIG. 4 is a ride side elevational view thereof.
FIG. 5 is a left side elevational view thereof.
FIG. 6 is a front elevational view thereof; and,
FIG. 7 is a right rear elevational view thereof.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.