Book departure (6)
I believe the answer is:
exodus
This clue is a double definition.
'book' is the first definition.
(I know that Exodus is a book)
'departure' is the second definition.
(I've seen this before)
(Other definitions for exodus that I've seen before include "Mass flight" , "Mass departure as in Old Testament's second Book" , "Film of book" , "Departure of the holy book" , "Second book of the Old Testament" .)