Disturbing dream of a black horse (9)
I believe the answer is:
nightmare
'disturbing dream' is the definition.
'nightmare' can be an answer for 'dream' (nightmare is a kind of dream). I'm unsure of the 'disturbing' bit.
'a black horse' is the wordplay.
'a black' becomes 'night' (night can mean as black as night).
'horse' becomes 'mare' (mare is a kind of horse).
'night'+'mare'='NIGHTMARE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "It makes one restless" , "Scary nocturnal fantasy" , "Awful experience" , "He joins migrant in scary illusion" , "hellish event" .)