Love god's achy back (4)
I believe the answer is:
eros
'love god's' is the definition.
(Greek god of love)
'achy back' is the wordplay.
'achy' becomes 'sore' (I've seen this in another clue).
'back' shows that the letters should be reversed in order.
'sore' in reverse letter order is 'EROS'.
(Other definitions for eros that I've seen before include "Goddess" , "In Greek myth, the god of love" , "Greek equivalent to Roman Cupid" , "London landmark" , "asteroid four hundred and thirty three" .)