Crossword Genius

Book lasting a year? (8)

Ross

I believe the answer is:

twelvemo

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'book' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I don't see?

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

Can you help me to learn more?

(Another definition for twelvemo that I've seen is " Size of book".)

I've seen this clue in The Times.
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