Crossword Genius

Question over penning occult knowledge in book (6)

Ross

I believe the answer is:

quarto

I'm a little stuck... Click here to teach me more about this clue!

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

'question over penning occult knowledge' is the wordplay.
'question' becomes 'qu' (abbreviation).
'over' becomes 'o' (cricket abbreviation).
'penning' is an insertion indicator.
'occult knowledge' becomes 'art' (art is a kind of knowledge. I am not sure about the 'occult' bit.).
'qu'+'o'='quo'
'quo' placed around 'art' is 'QUARTO'.

'in' acts as a link.

Can you help me to learn more?

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

I've seen this clue in The Guardian.
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