London statue painful to knock over (4)
I believe the answer is:
eros
'london statue' is the definition.
(statue of Anteros which is often called Eros)
'painful to knock over' is the wordplay.
'painful' becomes 'sore' (similar in meaning).
'to knock over' says the letters should be written in reverse.
'sore' written backwards gives 'EROS'.
(Other definitions for eros that I've seen before include "Sore about love god" , "Rose for the love-god" , "Greek god of love - see statue in Piccadilly" , "divinity" , "Greek equivalent to Roman Cupid" .)