Keen for operation for joint (4)
I believe the answer is:
knee
'for joint' is the definition.
(knee joint is a kind of joint)
'keen for operation' is the wordplay.
'keen' anagrams to 'KNEE'.
However, I'm not clear how this is indicated.
Can you help me to learn more?
(Other definitions for knee that I've seen before include "for going on begging" , "one is bent on submission" , "proposal might be made on one" , "It jerks reactively" , "Wounded in Indian Wars action" .)