The ornamental design for an apron, as shown and described.
1. Apron
1.1: Front
1.2: Front view with waist panel lifted
1.3: Back
The design is for an apron as shown in the three reproductions; the very light dotted lines, close to the darker edges of the apron, indicate stitching that forms no part of the claimed design; the broken lines indicating portions of the apron form no part of the claimed design, however the surface of the apron adjacent to top edge of the apron and around the front pockets is part of the claimed design, and the straps are shown with breaks and no portion of the straps shown within the breaks form part of the claimed design.