The ornamental design for pants with an undersidezipper, as shown and described.
FIG. 1 is a front perspective view of pants with an underside zipper showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof.
FIG. 7 is a bottom view thereof;
FIG. 8 is a front perspective view of the pants with an underside zipper depicted with zipper cover flaps turned outward;
FIG. 9 is a front view thereof;
FIG. 10 is a rear view thereof;
FIG. 11 is a left side view thereof;
FIG. 12 is a right side view thereof;
FIG. 13 is a top view thereof; and,
FIG. 14 is a bottom view thereof.
The broken lines shown in the Figures is included for the purpose of illustrating a portion of the pants with an underside zipper that forms no part of the claimed design.