You won't find this in the North Atlantic! (4)
I believe the answer is:
that
'you won't find this in the north atlantic' is the definition.
The definition suggests an adverb which matches the answer.
'in the north atlantic' is the wordplay.
'in' says the answer is hidden in the clue.
'THAT' is hidden within 'north atlantic'.
Can you help me to learn more?
(Other definitions for that that I've seen before include "The (one) there" , "Just like --; -- is to say" , "See 5" , "Take ..., English pop band" , "Yon" .)