Folded sheet of paper (5)
I believe the answer is:
folio
'folded sheet of paper' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for folio that I've seen before include "Folded sheet - or book made from such" , "Book size" , "Book made of folded sheets" , "Sheet of paper folded to make two book leaves" , "Manuscript sheet" .)