Sign of the future (4)
I believe the answer is:
'sign of the future' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omen that I've seen before include "Harbinger, portent" , "Augury, portent" , "Straw in the wind" , "Event regarded as a prophetic sign of good or evil" , "Sign of something to come" .)