Sign of what's to come? (4)
I believe the answer is:
omen
'sign of what's to come' 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 "Forecast" , "Forewarning, harbinger" , "Sign of something to come" , "Warning token" , "Sign of a future event, good or bad" .)