Upset about love (4)
I believe the answer is:
eros
'love' is the definition.
(Greek god of love)
'upset about' is the wordplay.
'upset' becomes 'sore' (I've seen this in other clues).
'about' says the letters should be written in reverse.
'sore' backwards is 'EROS'.
(Other definitions for eros that I've seen before include "Greek god of love - see statue in Piccadilly" , "God of love - example of 1 down" , "Arrow-shooting god" , "Statue of love-god in Piccadilly Circus in London" , "One worshipped" .)