The ornamental design for a casting for a fixture, as shown and described.
FIG. 1 is a perspective view of a casting for a fixture according to the present invention, an exemplary fixture attached thereto being shown in broken lines for illustrative environmental purposes only and thus not forming a part of the claimed design;
FIG. 2 is a top plan view thereof, the bottom plan view being plain and unornamented and thus not forming part of the claimed design; and,
FIG. 3 is a side elevational view thereof, all sides being substantially symmetrical.