Then a grim disturbing incubus (9)
I believe the answer is:
nightmare
'incubus' is the definition.
(synonyms)
'then a grim disturbing' is the wordplay.
'disturbing' indicates an anagram.
'then'+'a'+'grim'='thenagrim'
'thenagrim' anagrammed gives 'NIGHTMARE'.
(Other definitions for nightmare that I've seen before include "Scary fantasy while asleep" , "He joins migrant in scary illusion" , "terrible experience" , "Frightening dream" , "Frightening nocturnal fantasy" .)