Mounting an Atlantic isle (9)

I believe the answer is:

ascension

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This might be a double definition.

'mounting an atlantic' is the first definition.
'ascension' can be an answer for 'mounting' (thesaurus). I am not sure about the 'an atlantic' bit.

'isle' is the second definition.
(I've seen this before)

Can you help me to learn more?

(Other definitions for ascension that I've seen before include "distance in hours measured by this, right?" , "Canonise on the way up to heaven" , "religious feast" , "A movement upward" , "Canonises on Christ's way to heaven" .)

I've seen this clue in the Sydney Morning Herald.

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