Book Suzi? Time to move on (6)

I believe the answer is:

quarto

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'book' is the definition.
(I know that quarto is a type of book)

I don't understand the rest of the clue.

(Other definitions for quarto that I've seen before include "Ten-by-eight inch paper" , "Book size resulting in folding one sheet four times" , "Twice-folded paper size" , "Publishing term, four leaves are formed from one sheet" , "Book page size" .)

I've seen this clue in The Guardian.

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