The ornamental design for a pocket for a bag, as shown and described.
FIG. 1 is a perspective view of a a pocket for a bag;
FIG. 2 is a front view thereof;
FIG. 3 is a detail view of the pattern indicated in FIG. 2; and,
FIG. 4 is a front view of a second embodiment of the design shown in FIG. 1.
The broken lines showing a handbag are for the purposes of illustrating environment and form no part of the claimed design. It should be understood that the pattern of FIG. 3 is continuous on the surfaces of the individual sections of the pocket on which it is depicted in FIG. 2 and that same section of the pocket for a bag in FIG. 1, and the pattern of FIG. 3 is not found elsewhere on the embodiments of a design for a pocket for a bag. The stippling in the view of FIG. 4 and to a lesser extent in FIG. 2 is meant to represent the surface of the pocket and is not meant to convey a texture or pattern.