Sign of something about to happen (7)
I believe the answer is:
portent
'sign of something about to happen' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for portent that I've seen before include "Augury" , "Prophetic sign" , "Sign, omen" , "Sign or warning of an event likely to happen" , "Foreboding" .)