Heaving ocean, small boat (5)
I believe the answer is:
canoe
'small boat' is the definition.
(canoe is a kind of small boat)
'heaving ocean' is the wordplay.
'heaving' is an anagram indicator (letters thrown or heaved around).
'ocean' anagrammed gives 'CANOE'.
(Other definitions for canoe that I've seen before include "Kayak, for instance" , "Narrow paddle boat" , "Light, narrow open boat" , "Light boat propelled with paddle" , "Ocean (anag)" .)