The ornamental design for a stand-up clock, as shown and described.
FIG. 1 is a front perspective view of a stand-up clock showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The structural features and indicia depicted by broken lines in the drawings have been shown for the purpose of illustrating portions of the stand-up clock that form no part of the claimed design.