The ornamental design for a portion of a stackable furnishing, as shown and described.
FIG. 1 is a perspective view of a portion of a stackable furnishing showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dash-dash-dash broken lines shown in the drawings form no part of the claimed design. The dash-dot-dash broken lines shown in the drawings represent boundaries of the claim and form no part of the claimed design.