Greek god is angry when upset (4)
I believe the answer is:
eros
'greek god' is the definition.
(Eros is an example)
'angry when upset' is the wordplay.
'angry' becomes 'sore' (similar in meaning).
'when upset' says the letters should be written in reverse (in a down clue - set, written, up).
'sore' backwards is 'EROS'.
'is' acts as a link.
(Other definitions for eros that I've seen before include "very centre of London?" , "God of love; life instinct (Freud)" , "Cupid, son of Aphrodite" , "capital statue" , "Form of love" .)