Cupid has a painful back (4)
I believe the answer is:
eros
'cupid' is the definition.
(I've seen this before)
'a painful back' is the wordplay.
'a painful' becomes 'sore' (I've seen this before).
'back' is a reversal indicator.
'sore' reversed gives 'EROS'.
'has' acts as a link.
(Other definitions for eros that I've seen before include "Form of love" , "Love-god's statue in Piccadilly" , "Circus boy" , "Roman god of love, Cupid" , "Cupid's Greek alternative" .)