One may be bent back, needing to be shortened (4)
I believe the answer is:
knee
'one may be bent' is the definition.
(a leg bends at the knee)
'back needing to be shortened' is the wordplay.
'to be shortened' says the answer is hidden in the clue (a shortened or reduced extract from the clue).
'KNEE' is hidden in the letters of 'back needing'.
(Other definitions for knee that I've seen before include "Human joint" , "Hinged joint" , "Major joint in leg" , "Leg hinge" , "Thigh/shin connector" .)