The ornamental design for a handle for a window covering, as shown and described.
FIG. 1 is a top perspective view of a handle for a window covering showing our design in a position of use;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right side view thereof, which is a mirror image of the left side view; and,
FIG. 6 is a bottom perspective view thereof.
The broken lines in FIG. 1 illustrating a rail on which the handle for a window covering is mounted depicts environmental structure and forms no part of the claimed design.