Like dark clouds (7)
I believe the answer is:
ominous
'like dark clouds' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ominous that I've seen before include "Foreboding, threatening" , "Portending evil" , "Foreboding, menacing" , "Looking like trouble ahead" , "Threatening, doesn't look good" .)