Dummy's perch (4)
I believe the answer is:
'dummy's perch' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knee that I've seen before include "It jerks reactively" , "Leg hinge" , "Technical name for this joint is genu" , "Leg bender" , "Where the femur meets the tibia" .)