Mass emigration (6)
I believe the answer is:
exodus
'mass emigration' 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 "Flight of the Israelites out of Egypt, led by Moses" , "Biblical book - going out" , "Departure of the Israelites from Egypt, led by Moses" , "Journey of escape" , "A mass departure of people, as in Bible" .)