Ocean unsuitable for small vessel (5)
I believe the answer is:
canoe
'small vessel' is the definition.
'canoe' can be an answer for 'vessel' (canoe is a kind of vessel). I am not certain of the 'small' bit.
'ocean unsuitable' is the wordplay.
'unsuitable' indicates anagramming the letters (letters not in suitable order).
'ocean' is an anagram of 'CANOE'.
'for' is the link.
Can you help me to learn more?
(Other definitions for canoe that I've seen before include "Light, narrow open boat" , "Light boat propelled with paddle" , "Small boat for paddling" , "Simple boat" , "A dugout" .)