An omen (6)
I believe the answer is:
augury
'an omen' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for augury that I've seen before include "Portent or harbinger" , "Omen or its interpretation" , "Prophesy" , "Portent or prophecy" , "Divination, prediction" .)