The ornamental design for a clasp for a bag, as shown and described.
1. Clasp for a bag
1.1 : Perspective
1.2 : Front
1.3 : Right
1.4 : Left
1.5 : Top
1.6 : Bottom
1.7 : Another Perspective, shown in an open condition of use.
1.8 : Another Perspective, shown enlarged with the clasp in a closed condition of use.
The dashed broken lines in the Figures depict portions of the clasp for a bag that form no part of the claimed design. The dot-dash broken lines in Figure 1.8 depicts the bounds of the enlarged view and form no part of the claimed design.