Keep page attached to book (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep is a more specific form of enter)
'page attached to book' is the wordplay.
'page' becomes 'p'.
'attached to' says to put letters next to each other.
'book' becomes 're-serve' (as in booking or reserving in advance).
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "Keep out of harm's way" , "Eg, jam" , "Particular interest" , "defend" , "Keep safe or in unchanged condition" .)