God is grievously upset (4)
I believe the answer is:
eros
'god' is the definition.
(I know that Eros is a Greek deity)
'grievously upset' is the wordplay.
'grievously' becomes 'sore' (sore can mean grievously or severely).
'upset' is a reversal indicator.
'sore' back-to-front is 'EROS'.
'is' is the link.
(Other definitions for eros that I've seen before include "In Greek myth, the god of love" , "Form of love" , "Love-god's statue in Piccadilly" , "Greek god of sexual love, Cupid to the Romans" , "Greek equivalent to Roman Cupid" .)