Piece of metal used for part of joint (5)
I believe the answer is:
talus
'part of joint' is the definition.
(the talus is an ankle bone)
'piece of metal used for' is the wordplay.
'piece of' indicates a hidden word.
'TALUS' is hidden within 'metal used for'.
(Other definitions for talus that I've seen before include "Foot's part" , "Yarns" , "Ankle bone" , "Ankle-bone" .)