The ornamental design for a clipboard, as shown and described.
FIG. 1 is a front perspective view of a clipboard showing the new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom thereof;
FIG. 4 is a front thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a right side thereof;
FIG. 7 is a left side thereof; and,
FIG. 8 is a front perspective view thereof showing the clipboard in its open configuration.
The broken lines in FIGS. 3, 5, 6, and 7 showing portions of the clipboard are included for the purpose showing features that form no part of the claimed design.