I harm gent suffering hallucination (9)
I believe the answer is:
nightmare
'hallucination' is the definition.
(a nightmare is a hallucinatory phenomenon)
'i harm gent suffering' is the wordplay.
'suffering' indicates anagramming the letters.
'i'+'harm'+'gent'='iharmgent'
'iharmgent' is an anagram of 'NIGHTMARE'.
(Other definitions for nightmare that I've seen before include "Horrifying dream" , "that might upset the rest" , "hellish event" , "Terrifying experience" , "Scary fantasy while asleep" .)