Cupid's painful back (4)
I believe the answer is:
eros
'cupid's' is the definition.
(thesaurus)
'painful back' is the wordplay.
'painful' becomes 'sore' ('sore' can be a synonym of 'painful').
'back' says the letters should be written in reverse.
'sore' reversed gives 'EROS'.
(Other definitions for eros that I've seen before include "Form of love" , "Cupid's Greek alternative" , "Arrow-shooting god" , "Piccadilly 14 ac statue" , "One worshipped" .)