Save page on book (8)
I believe the answer is:
preserve
'save' is the definition.
(synonyms)
'page on book' is the wordplay.
'page' becomes 'p'.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'book' becomes 're-serve' (as in booking or reserving in advance).
'p'+'reserve'='PRESERVE'
(Other definitions for preserve that I've seen before include "Toast option" , "Keep - pickle - store" , "kipper?" , "Maintain, protect from decay" , "Area of activity" .)