The ornamental design for a frontportion of a retaining wall, as shown and described.
FIG. 1 shows a perspective view of a retaining wall;
FIG. 2 shows a front view of the retaining wall illustrated in FIG. 1;
FIG. 3 shows a back view of the retaining wall illustrated in FIG. 1;
FIG. 4 shows a right side view of the retaining wall illustrated in FIG. 1;
FIG. 5 shows a left side view of the retaining wall illustrated in FIG. 1;
FIG. 6 shows a top view of the retaining wall illustrated in FIG. 1; and,
FIG. 7 shows a bottom view of the retaining wall illustrated in FIG. 1.
The broken lines in the drawing views are included for the purpose of illustrating portions of the retaining wall that form no part of the claimed design.