Dark cloud, maybe (4)
I believe the answer is:
omen
'dark cloud maybe' 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 "Sign of something about to happen" , "Forewarning, harbinger" , "Harbinger, portent" , "Gloomy sign" , "Portent of things to come" .)