Book substitute (7)
I believe the answer is:
reserve
I believe this clue is a double definition.
'book' is the first definition.
(as in booking or reserving in advance)
'substitute' is the second definition.
(I know that substitute can be written as reserve)
(Other definitions for reserve that I've seen before include "cool" , "Order" , "Keep back - book" , "Book - substitute player" , "Twelfth man < 's coolness" .)