Evil consumes good omen (4)
I believe the answer is:
sign
'omen' is the definition.
(I've seen this before)
'evil consumes good' is the wordplay.
'evil' becomes 'sin' (I've seen this before).
'consumes' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'sin' going around 'g' is 'SIGN'.
(Other definitions for sign that I've seen before include "Public notice; Leo, say" , "Show" , "Ram" , "Authorise with a name" , "Communicate silently" .)