The ornamental design for a portion of a skylight, as shown and described.
FIG. 1 is a top-front perspective view of a portion of a skylight showing our new design;
FIG. 2 is a left-front perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a left side view thereof.
The broken lines immediately adjacent to the shaded area represent unclaimed boundaries of the design. The broken lines showing the remainder of the portion of a skylight are for environmental purposes only and form no part of the claimed design.