Foreboding sign (4)
I believe the answer is:
omen
'foreboding sign' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for omen that I've seen before include "Portent of evil" , "Event regarded as a prophetic sign of good or evil" , "Harbinger, augury" , "Sign of things to come" , "Foreboding" .)