Be out till morning, perhaps (5)

I believe the answer is:

sleep

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'be' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I can't explain the rest of the clue.

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(Other definitions for sleep that I've seen before include "Be dormant" , "Slumber" , "Wayne _; former Royal Ballet principal dancer" , "Drop off" , "Produced (egg)" .)

I've seen this clue in the Evening Standard.

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