Jack needs to display a joint (4)
I believe the answer is:
knee
'a joint' is the definition.
(I know that knee is a type of joint)
'jack needs to display' is the wordplay.
'to display' indicates the answer is hidden within the clue (I've seen 'displayed' mean this (the answer is displayed within the clue)).
'KNEE' is hidden within 'jack needs'.
(Other definitions for knee that I've seen before include "one is bent on submission" , "proposal might be made on one" , "middle part of leg" , "Genuflector?" , "Joint protected by the patella" .)