Part of link needing a joint (4)
I believe the answer is:
knee
'joint' is the definition.
(knee is a kind of joint)
'part of link needing a' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'KNEE' can be found hidden inside 'link needing a'.
(Other definitions for knee that I've seen before include "proposal might be made on one" , "Leg hinge" , "Hinge between the thigh and the shin" , "Flexible joint" , "Joint of the body" .)