Book to keep? (7)
I believe the answer is:
reserve
This clue is a double definition.
'book' is the first definition.
(as in booking or reserving in advance)
'keep?' is the second definition.
(I know that keep is a more specific form of the action reserve)
'to' is the link.
(Other definitions for reserve that I've seen before include "ice" , "Kept back stock" , "Wine kept" , "man on bench?" , "Lack of openness" .)