Dark horse in torment (9)
I believe the answer is:
nightmare
'torment' is the definition.
(thesaurus)
'dark horse' is the wordplay.
'dark' becomes 'night' (it's dark at nighttime).
'horse' becomes 'mare' (mare is a kind of horse).
'night'+'mare'='NIGHTMARE'
'in' is the link.
(Other definitions for nightmare that I've seen before include "Frightening nocturnal fantasy" , "dreadful experience" , "Scary fantasy while asleep" , "He joins migrant in scary illusion" , "hellish event" .)