The ornamental design for a part of a filter, as shown and described.
FIG. 1 is a top perspective view of a part of a filter;
FIG. 2 is a top plan view of the part of a filter shown in FIG. 1;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines depict portions of the part of a filter that form no part of the claimed design.