Keen to carve a joint (4)
I believe the answer is:
'a joint' is the definition.
(I know that knee is a type of ginglymoid joint)
'keen to carve' is the wordplay.
'to carve' is an anagram indicator (I've seen 'carving' mean this).
'keen' is an anagram of 'KNEE'.
(Other definitions for knee that I've seen before include "Hinge between the thigh and the shin" , "one is bent on submission" , "proposal might be made on one" , "Wounded in Indian Wars action" , "Leg hinge" .)