Angry when London statue's toppled (4)
I believe the answer is:
sore
'angry' is the definition.
('sore' can be similar in meaning to 'angry')
'london statue's toppled' is the wordplay.
'london statue' becomes 'eros' (statue of Anteros which is often called Eros).
'toppled' says the letters should be written in reverse (in a down clue, letters go up).
'eros' written backwards gives 'SORE'.
'when' acts as a link.
(Other definitions for sore that I've seen before include "Painful or tender" , "Painful spot" , "Hurt" , "Boil" , "source of distress" .)