Hell's dark horse (9)
I believe the answer is:
nightmare
'hell's' is the definition.
(thesaurus)
'dark horse' is the wordplay.
'dark' becomes 'night' (night is a kind of dark).
'horse' becomes 'mare' (mare is a kind of horse**).
'night'+'mare'='NIGHTMARE'
(Other definitions for nightmare that I've seen before include "Bad dream" , "Frightening dream" , "dreadful experience" , "Scary nocturnal fantasy" , "Horrible experience" .)