Case includes North Atlantic (4)
I believe the answer is:
pond
'atlantic' is the definition.
(as in 'across the pond')
'case includes north' is the wordplay.
'case' becomes 'pod' (pod is a kind of case).
'includes' indicates putting letters inside.
'north' becomes 'N' (abbreviation).
'pod' going around 'n' is 'POND'.
(Other definitions for pond that I've seen before include "Small area of still water" , "Himalayan country" , "Small body of water" , "Possible site for weed" , "Small lake" .)