At sea once, a small boat (5)
I believe the answer is:
canoe
'small boat' is the definition.
(canoe is a kind of small boat)
'at sea once a' is the wordplay.
'at sea' is an anagram indicator.
'once'+'a'='oncea'
'oncea' anagrammed gives 'CANOE'.
(Other definitions for canoe that I've seen before include "Small boat with paddles" , "Boat one paddles" , "Light boat propelled with paddle" , "Small boat for paddling" , "Boat (that can be dug out!)" .)