Keep page on top of book (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep is a kind of enter)
'page on top of book' is the wordplay.
'on top of' indicates taking the first letters (I've seen 'on top' mean this).
'book' becomes 're-serve' (as in booking or reserving in advance).
The initial letter of 'page' is 'p'.
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "Refuse to give up" , "defend" , "Keep safe from spoilage" , "Keep from decay in perverse way" , "Eg, jam" .)