Book is great departure (6)
I believe the answer is:
exodus
This clue could be a double definition.
'book' is the first definition.
(I know that Exodus is an book)
'great departure' is the second definition.
'exodus' can be an answer for 'departure' (I've seen this in another clue). I am not certain of the 'great' bit.
'is' is the link.
Can you help me to learn more?
(Other definitions for exodus that I've seen before include "Act of mass departure" , "Mass leaving" , "A great departure or outflow of people" , "OT book - flight" , "A great departure or dispersal" .)