Crossword Genius

Unlikely winner's 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 clue might be a double definition.

'unlikely winner's' is the first definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for dark horse that I've seen before include "Little is known of its properties" , "unexpectedly successful entry" , "Competitor whose abilities are unknown" , "it may be foolish to discount" , "Competitor of unknown capabilities" .)

I've seen this clue in The Sun.
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