A bone can keep breaking! (7)
I believe the answer is:
'a bone' is the definition.
(I know that kneecap is a type of bone)
'can keep breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'cankeep' anagrammed gives 'KNEECAP'.
(Other definitions for kneecap that I've seen before include "Joint protection" , "one going on bender?" , "Horribly punish" , "part of joint" , "The informal patella (7)" .)