The ornamental design for a clasp, as shown and described.
FIG. 1 is a perspective view of a clasp according to the design;
FIG. 2 is a front view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a perspective view of the clasp in an open configuration, with the two elements of the clasp separated between each other following a 90° rotation of the two vertical bars of the letters “F” on the front of the clasp; and,