The ornamental design for a rearportion for an automobile, as shown and described.
FIG. 1 is a front perspective view of a rear portion for an automobile showing our new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The claimed design comprises the shape and configuration of the relatively dark portions in the figures surrounded by broken lines. The showing of the automobile in the relatively light shaded contrast found outside the broken lines forms no part of the claimed design.