Rare deformity of the back (4)
I believe the answer is:
rear
'the back' is the definition.
('rear' can be a synonym of 'back')
'rare deformity' is the wordplay.
'deformity' is an anagram indicator.
'rare' is an anagram of 'REAR'.
'of' is the link.
(Other definitions for rear that I've seen before include "Rump" , "Rise up at the back" , "what can be brought up, finally" , "Hitchcock film set in one room" , "Bring up or stand up on hind legs" .)