Book to lay aside (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)
'lay aside' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for reserve that I've seen before include "not in the front line" , "put to one side" , "Modesty- store" , "Savings - book" , "Keep for future use" .)