Omen: good beset by evil (4)
I believe the answer is:
sign
'omen' is the definition.
(thesaurus)
'good beset by evil' is the wordplay.
'good' becomes 'g' (abbreviation).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'evil' becomes 'sin' (I've seen this before).
'g' inserted into 'sin' is 'SIGN'.
(Other definitions for sign that I've seen before include "add name" , "Signal, gesture" , "Ram" , "Token - symbol - omen - write one's name" , "Endorse" .)