Circus figure's painful back (4)
I believe the answer is:
eros
'circus figure's' is the definition.
(common name of statue at Piccadilly Circus)
'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'.
(Other definitions for eros that I've seen before include "Group of instincts" , "asteroid four hundred and thirty three" , "Asteroid - Greek god of love" , "Form of love" , "God of love - example of 1 down" .)