Threatening, ominous (8)
I believe the answer is:
menacing
This clue is a double definition.
'threatening' is the first definition.
(I know that threatening can be written as menacing)
'ominous' is the second definition.
(I know that ominous can be written as menacing)
(Other definitions for menacing that I've seen before include "Ominous" , "Threatening" , "Suggesting danger" , "Intimidating" , "Scary" .)