It's just between Canada and Britain (8)
I believe the answer is:
atlantic
'britain' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for atlantic that I've seen before include "Latin cat found, strangely, in the ocean" , "One of earth's oceans" , "Western ocean." , "What lies to the east of 10" , "main" .)