Crossword Genius

Having been run over by a train, is unconscious (7)

Ross

I believe the answer is:

sleeper

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

'unconscious' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sleeper that I've seen before include "Dormant potential spy" , "Rail track supporter" , "Earring" , "Spy; part of railway" , "Timber beam" .)

I've seen this clue in the Evening Standard.
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