Crossword Genius

Spy who’s still in bed? (7)

Ross

I believe the answer is:

sleeper

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'spy' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for sleeper that I've seen before include "One who slumbers" , "Railway tie" , "Supporting block on railway track" , "Train with berths" , "Long-inactive secret agent" .)

I've seen this clue in The Times.
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