Sea water or where it usually comes from (4,6)

I believe the answer is:

main source

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'where it usually comes from' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'sea water' is the wordplay.
'sea' becomes 'main' (synonyms).
'water' becomes 'source' (I can't explain this - if you can you should believe this answer much more).
'main'+'source'='MAIN SOURCE'

'or' is the link.

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