Information in folio book (5)
I believe the answer is:
facts
'information' is the definition.
(I've seen this before)
'folio book' is the wordplay.
'folio' becomes 'f' (abbreviation listed in Chambers).
'book' becomes 'acts' (book of the Bible).
'f'+'acts'='FACTS'
'in' is the link.
(Other definitions for facts that I've seen before include "these cannot be disputed" , "hard information" , "Truths" , "Pieces of information" , "Realities and certainties" .)