Dark horse appearing only in sleep (9)
I believe the answer is:
nightmare
'only in sleep' is the definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps there's a link between them 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'
'appearing' is the link.
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "dreadful experience" , "terrible experience" , "that might upset the rest" , "Scary nocturnal fantasy" , "Horrible experience" .)