The main split between Britain and the US (8,5)

I believe the answer is:

atlantic ocean

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'the main' is the definition.
(Atlantic Ocean is an example)

I cannot understand how the remainder of the clue works.

(Other definitions for atlantic ocean that I've seen before include "Large sea" , "link between Europe and America" , "Large expanse of water" , "It includes Sargasso sea" .)

I've seen this clue in The Globe and Mail.

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