Patella joint (4)
I believe the answer is:
knee
'patella joint' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for knee that I've seen before include "Thigh/shin connector" , "This is capped by the patella" , "Technical name for this joint is genu" , "one is bent on submission" , "Hinge joint" .)