Forewarning
I believe the answer is:
omen
'forewarning' 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 "Sign of a future event, good or bad" , "Warning token" , "Portent, prophetic indicator" , "Event regarded as a prophetic sign of good or evil" , "Sign of things to come" .)