Dark horse disturbing one's sleep? (9)
I believe the answer is:
nightmare
'disturbing one's sleep?' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'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'
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "Horrifying experience" , "Horrifying dream" , "Bad dream" , "terrible experience" , "It makes one restless" .)