One of many across the tracks (7)

I believe the answer is:

sleeper

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I cannot understand how the rest of the clue works.

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(Other definitions for sleeper that I've seen before include "Kipper" , "Rail supporter; secret agent" , "One having D [DREAM]" , "Express tears over one" , "Long-inactive secret agent" .)

I've seen this clue in the Evening Standard.

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