Book perhaps quietly turned down (6)

I believe the answer is:

volume

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'book perhaps' is the definition.
(book is a kind of volume)

I can't explain the rest of the clue.

(Other definitions for volume that I've seen before include "Publication" , "size" , "Book - measure of loudness" , "Magnitude of sound" , "reading matter?" .)

I've seen this clue in the Evening Standard.

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