Migration (6)
I believe the answer is:
exodus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">migration</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for exodus that I've seen before include "8, Book 2" , "A great departure or dispersal" , "Massive outflow of people" , "Journey of escape" , "A mass departure as of Jews from Egypt (6)" .)