Joint protected by the patella (4)
I believe the answer is:
knee
'joint protected by the patella' 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" , "Hinge between the thigh and the shin" , "Leg?joint" , "Technical name for this joint is genu" , "It jerks reactively" .)