Book a flight (6)
I believe the answer is:
exodus
I believe this is a double definition.
'book' is the first definition.
(book of the Bible)
'a flight' is the second definition.
(I know that exodus is a type of flight)
(Other definitions for exodus that I've seen before include "Departure of the holy book" , "Journey of escape" , "Massive outflow of people" , "A great departure as in the Bible" , "Mass migration (of a people)" .)