Keen to change the joint (4)
I believe the answer is:
knee
'the joint' is the definition.
(knee is a kind of joint)
'keen to change' is the wordplay.
'to change' indicates an anagram.
'keen' with letters rearranged gives 'KNEE'.
(Other definitions for knee that I've seen before include "Lower limb joint" , "Human joint" , "Leg hinge" , "This is capped by the patella" , "It jerks reactively" .)