A bone can keep breaking! (7)
I believe the answer is:
kneecap
'a bone' is the definition.
(kneecap is a kind of bone)
'can keep breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'can'+'keep'='cankeep'
'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)" .)