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' is an anagram indicator.
'cankeep' with letters rearranged 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)" .)