Black horse in bad dream (9)
I believe the answer is:
nightmare
'bad dream' is the definition.
(I've seen this in another clue)
'black horse' is the wordplay.
'black' becomes 'night' (I've seen this before).
'horse' becomes 'mare' (mare is a kind of horse).
'night'+'mare'='NIGHTMARE'
'in' acts as a link.
(Other definitions for nightmare that I've seen before include "Bad dream" , "Very unpleasant experience" , "causing distress to the Lord Chancellor?" , "Scary fantasy while asleep" , "It makes one restless" .)