Painful back for love god (4)
I believe the answer is:
eros
'love god' is the definition.
(I've seen this before)
'painful back' is the wordplay.
'painful' becomes 'sore' ('sore' can be a synonym of 'painful').
'back' is a reversal indicator.
'sore' in reverse letter order is 'EROS'.
'for' acts as a link.
(Other definitions for eros that I've seen before include "I'm in a circus in London" , "Greek god of love - see statue in Piccadilly" , "capital statue" , "Arrow-shooting god" , "Greek love-god" .)