Crossword Genius

Historic charter negotiated by seamen (8)

Ross

I believe the answer is:

atlantic

I'm a little stuck... Click here to teach me more about this clue!

'historic' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'charter negotiated by seamen' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'at' and 'at' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for atlantic that I've seen before include "Latin cat found, strangely, in the ocean" , "Great sea" , "What lies to the east of 10" , "UK's nearest ocean" , "Ocean from Arctic to Antarctic" .)

I've seen this clue in the Evening Standard.
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