Ocean unsuitable for such a vessel (5)
I believe the answer is:
canoe
'such a vessel' is the definition.
(canoe is a kind of vessel)
'ocean unsuitable' is the wordplay.
'unsuitable' is an anagram indicator (letters not in suitable order).
'ocean' anagrammed gives 'CANOE'.
'for' is the link.
(Other definitions for canoe that I've seen before include "Hunting vessel originating in Haiti" , "Light paddle-propelled boat" , "Kayak, for instance" , "Boat that's paddled" , "Small boat for paddling" .)