Wail when displacing back joint (4)
I believe the answer is:
knee
'joint' is the definition.
(knee is a kind of joint)
I don't understand the rest of the clue.
(Other definitions for knee that I've seen before include "Joint of the body" , "Technical name for this joint is genu" , "Where the femur meets the tibia" , "one is bent on submission" , "Wounded in Indian Wars action" .)