Going out for a book (6)
I believe the answer is:
exodus
I believe this is a double definition.
'going out' is the first definition.
(I've seen this before)
'book' is the second definition.
(book of the Bible)
'for a' is the link.
(Other definitions for exodus that I've seen before include "Mass departure of people, as in 2nd Book of O. T." , "Mass flight" , "8, Book 2" , "A great departure or dispersal" , "Flight of the Israelites out of Egypt, led by Moses" .)