Former partner raised fare to go on American flight (6)
I believe the answer is:
Here is my best explanation:
'flight' is the definition.
(exodus is a kind of flight)
'former partner raised fare to go on american' is the wordplay.
'former partner' becomes 'ex'.
'raised' says the letters should be written in reverse.
'fare' becomes 'do' ('do' can be a synonym of 'fare').
'to go on' says to put letters next to each other (in a down clue, some letters go on top of others).
'american' becomes 'us'.
'do' reversed gives 'od'.
(Other definitions for exodus that I've seen before include "Biblical departure", "8, Book 2", "A mass departure, as in O.T. book", "sequel to first one", "Act of mass departure", "journey", "A mass departure of people, as in Bible", "Mass evacuation", "Mass leaving" and "Mass departure of people as of Jews from Egypt".)