Kept page folded finally under book (9)
I believe the answer is:
preserved
'kept' is the definition.
(preserving is a kind of keeping)
'page folded finally under book' is the wordplay.
'page' becomes 'p'.
'finally' suggests the final letters.
'under' is a charade indicator (letters next to each other).
'book' becomes 'reserve' ('reserve' can be a synonym of 'book'**).
The final letter of 'folded' is 'd'.
'd' after 'reserve' is 'reserved'.
'p'+'reserved'='PRESERVED'
(Other definitions for preserved that I've seen before include "Kept in an unaltered condition" , "Prevented from decay" , "Prevented from decaying or spoiling" , "Kept safe from harm" , "Maintained in an unaltered condition" .)