Cry out in sorrow when displacing back joint (4)
I believe the answer is:
knee
'joint' is the definition.
(I know that knee is a type of joint)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for knee that I've seen before include "Capped joint?" , "One bent in genuflection" , "It jerks reactively" , "Thigh/shin connector" , "Hinge between the thigh and the shin" .)