Joint protection once called in revolutionary gang (7)
I believe the answer is:
kneecap
'joint protection' is the definition.
(knee joint)
'called in revolutionary gang' is the wordplay.
'called' becomes 'nee' (nee may be written to show someone's name at birth).
'in' means one lot of letters goes inside another.
'revolutionary' says the letters should be written in reverse (letters revolve around).
'gang' becomes 'pack' (synonyms).
'pack' reversed gives 'kcap'.
'nee' inserted within 'kcap' is 'KNEECAP'.
'once' acts as a link.
(Other definitions for kneecap that I've seen before include "Horribly punish" , "one going on bender?" , "Joint protection" , "Bone" , "The informal patella (7)" .)