It's called the sack (3)

I believe the answer is:

bed

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'sack' is the definition.
('in the sack' means 'in bed')

I don't understand how the rest of the clue works.

(Other definitions for bed that I've seen before include "Bottom of the sea or river" , "might be laid here" , "Bottom < layer" , "Matrimonial duties" , "One with flower, sleeper, in" .)

I've seen this clue in the Evening Standard.

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