We claim the ornamental design for an insert for a trash can, as shown and described.
FIG. 1 is a front, top and side perspective view of the insert for a trash can embodying the design of the present invention.
FIG. 2 is a side elevational view thereof, the side opposite to the side shown being identical to the side shown in FIG. 2, the sides not shown in FIG. 1 being identical to the side shown in FIG. 2 without the figure shown in FIG. 2.