Mass departure (6)
I believe the answer is:
'mass departure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for exodus that I've seen before include "Mass departure as in Old Testament's second Book" , "8, Book 2" , "Second book of OT" , "Mass departure of people as of Jews from Egypt" , "Book following Genesis" .)