Some stock needed for joint (4)
I believe the answer is:
knee
'joint' is the definition.
(I know that knee is a type of synovial joint)
'some stock needed for' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'KNEE' is hidden within 'stock needed for'.
(Other definitions for knee that I've seen before include "proposal might be made on one" , "Hinge joint" , "Leg hinge" , "Leg?joint" , "It jerks reactively" .)