One may be in sea passage, heading off (4)
I believe the answer is:
isle
'one may be in sea' is the definition.
(an isle might be located in the sea)
'passage heading off' is the wordplay.
'passage' becomes 'aisle' (aisle can mean a path or passageway).
'heading off' means to remove the first letter.
'aisle' with its initial letter taken off is 'ISLE'.
(Other definitions for isle that I've seen before include "Land surrounded entirely by water" , "Detached place" , "-- of Dogs, area of London" , "Capri for one" , "Capri or Man, perhaps?" .)