Exodus was one of these with surging Red Sea put asunder finally (10)

I believe the answer is:

departures

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'these with surging red sea put asunder finally' is the definition.
The definition suggests an adverb but the answer is not.

'exodus was one' is the wordplay.
'exodus' becomes 'departure' (I've seen this before).
'was one' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'departure'+'s'='DEPARTURES'

'of' is the link.

Can you help me to learn more?

(Other definitions for departures that I've seen before include "leavings" , "changes" .)

I've seen this clue in The Guardian.

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