Sea vessel (4)
I believe the answer is:
ship
'sea vessel' is the definition.
(I know that ship is a type of sea vessel)
This is all the clue.
(Other definitions for ship that I've seen before include "Seagoing boat" , "In a word, final indication of rank or position" , "Navigable vessel" , "Large sailing vessel" , "Convey (goods); a vessel" .)