This behavior has been noted from many aquarists and has not been explained. Some clowns tug and pull on tentacles, and on occasion will vigorously rip off tentacles or tentacle tips. Larger anemones can recover from this abuse more easily than small anemones. You may try feeding your clown more frequently to see if this reduces the incidence of biting. However if the clown continues to bite and the anemone is stressed, you may have to remove one or the other from the tank. You may also want to run some fresh carbon to help absorb any mucus or chemicals seeping from the wounds of the anemone.
Thomas