Crossword Genius

Dark horse disturbing one's sleep? (9)

Ross

I believe the answer is:

nightmare

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'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" .)

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