The ornamental design for a onesie with a loop, as shown and described.
FIG. 1 is a front perspective view of the onesie with a loop;
FIG. 2 is a front elevation view of the onesie with a loop;
FIG. 3 is a rear elevation view of the onesie with a loop;
FIG. 4 is a left side elevation view of the onesie with a loop;
FIG. 5 is a right side elevation view of the onesie with a loop;
FIG. 6 is a top plan view of the onesie with a loop; and,
FIG. 7 is a bottom plan view of the onesie with a loop.
The partial human figure shown in broken lines is environmental subject matter included to illustrate how the design may appear when worn and forms no part of the claimed design. Both the figure on the front of the onesie with a loop and the shape attached to the loop are features of the article that form no part of the claimed design.