Book in fact unopened (4)
I believe the answer is:
ruth
'book' is the definition.
(I've seen this before)
'fact unopened' is the wordplay.
'fact' becomes 'truth' (truth is a kind of fact).
'unopened' means to remove the first letter.
'truth' with its first letter taken away is 'RUTH'.
'in' acts as a link.
(Other definitions for ruth that I've seen before include "She is followed by Samuel" , "Biblical Book with woman's name - a pity" , "Babe -, US baseball player" , "Babe -, US baseball star" , "Girl's name and Old Testament Book" .)