Historic charter between the Irish and the Americans? (8)

I believe the answer is:

atlantic

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'historic' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for atlantic that I've seen before include "Sea between Old and New Worlds" , "deep water" , "Latin cat strangely in the sea" , "What lies to the east of 10" , "One of earth's oceans" .)

I've seen this clue in the Evening Standard.

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