Part of a book once it's folded (5)

I believe the answer is:

folio

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

'part of a book' is the definition.
'folio' can be an answer for 'book' (I've seen this before). I'm unsure of the 'part of a' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for folio that I've seen before include "Leaf of paper" , "Book; wrapper for loose papers" , "Leaf of book - page number" , "outsize book" , "Book size" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app