Foreboding signs (5)
I believe the answer is:
'foreboding signs' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omens that I've seen before include "Prognostications" , "Auguries" , "Prophetic signs of good or evil" , "Harbingers, portents" , "Signs or warnings for the future" .)