I found a whole 3ds apple sometime ago at 3DCafe. To make it partially eaten, make a simmetrical lattice drawing a circle with irregular borders. You'll need more than one. Scale and position the lattices accordingly, and apply a yellowish material. Make the apple positive, the lattices negative, apply the lattices to simulate the eaten part and group. You may see the final result in one of my pics "Eve: Desire" in my gallery at Renderotica (search for nandus).