Crossword Genius

Surprising success for nightmare? (4,5)

Ross

I believe the answer is:

dark horse

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'surprising success' is the first definition.
The definition and answer can be both people as well as being singular nouns.
Maybe there's a link between them I don't understand?

'nightmare?' is the second definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for dark horse that I've seen before include "Competitor about whose abilities little is known" , "Competitor whose abilities are unknown" , "Brown heroin" , "Little-known competitor like shadowy equine" , "unexpectedly successful entry" .)

I've seen this clue in The Guardian.
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