Volume; reserve (4)
I believe the answer is:
book
This clue is a double definition.
'volume' is the first definition.
(I know that volume can be written as book)
'reserve' is the second definition.
(to book something is to reserve it)
(Other definitions for book that I've seen before include "strict regulations" , "Strict rules" , "Reserve - bets taken on a race" , "Paperback" , "Reserve (tickets)" .)