Crossword Genius

Wail when displacing back joint (4)

Ross

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" .)

I've seen this clue in The Times.
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