Keen to fix joint (4)
I believe the answer is:
knee
'joint' is the definition.
(knee is a kind of joint)
'keen to fix' is the wordplay.
'to fix' is an anagram indicator (I've seen 'fixed' mean this).
'keen' anagrammed gives 'KNEE'.
(Other definitions for knee that I've seen before include "Wounded in Indian Wars action" , "One bent in genuflection" , "middle part of leg" , "Human joint" , "proposal might be made on one" .)