Channel as start for sail across the Atlantic (5)
I believe the answer is:
sinus
'channel' is the definition.
(sinus is a kind of channel)
I can't explain the remainder of the clue.
(Other definitions for sinus that I've seen before include "Cavity linked to nose" , "Cavity within a bone" , "Space in temple" , "Channel between brain and skull" , "Nasal cavity" .)