Ocean unsuitable for this vessel (5)
I believe the answer is:
canoe
'this vessel' is the definition.
(canoe is a kind of vessel)
'ocean unsuitable' is the wordplay.
'unsuitable' indicates an anagram (letters not in suitable order).
'ocean' is an anagram of 'CANOE'.
'for' is the link.
(Other definitions for canoe that I've seen before include "Primitive boat" , "Narrow paddle boat" , "Ocean (anag)" , "Light, narrow open boat" , "Light boat propelled by paddle(s)" .)