Dark horses leaving town (5)

I believe the answer is:

sable

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'dark' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for sable that I've seen before include "Bales of valuable Russian fur" , "Fur - heraldic black" , "Dark brown fur of the marten" , "Adventurous courage" , "Black; antelope" .)

I've seen this clue in The Independent.

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