Gallons on board empty vessel (5)
I believe the answer is:
barge
'vessel' is the definition.
(barge is a kind of vessel)
'gallons on board empty' is the wordplay.
'gallons' becomes 'g'.
'on board' means one lot of letters goes inside another.
'empty' becomes 'bare' ('bare' can be similar in meaning to 'empty').
'g' going within 'bare' is 'BARGE'.
(Other definitions for barge that I've seen before include "Canal boat maybe" , "Long, flat-bottomed boat" , "Freight-carrying boat" , "Jostle; a boat" , "Push - boat" .)