Book store (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)
'store' is the second definition.
(a reserve is a store or supply of something)
(Other definitions for reserve that I've seen before include "ice" , "Savings - book" , "Conservation area" , "Hold back" , "Husband - substitute" .)