Ominous entity swallowing redhead, devoured (11)
I believe the answer is:
threatening
'ominous' is the definition.
('threatening' can be a synonym of 'ominous')
'entity swallowing redhead devoured' is the wordplay.
'entity' becomes 'thing' (thing is a kind of entity**).
'swallowing' means one lot of letters goes inside another.
'redhead' becomes 'r' (the head/first letter of 'red').
'devoured' becomes 'eaten' ('eaten' can be similar in meaning to 'devoured').
'r'+'eaten'='reaten'
'thing' enclosing 'reaten' is 'THREATENING'.
(Other definitions for threatening that I've seen before include "giving a warning for the future" , "Minatory" , "Dangerous" , "Scary" , "Putting at risk" .)