Passage south across the Atlantic (5)
I believe the answer is:
sinus
'passage' is the definition.
(I know that sinus is a type of canal)
'south across the atlantic' is the wordplay.
'south' becomes 'S' (abbreviation).
'across the atlantic' becomes 'in US' (in the United States).
's'+'inus'='SINUS'
(Other definitions for sinus that I've seen before include "snotty enclosure" , "Head space" , "Hole in the head" , "chamber" , "Channel between brain and skull" .)