Boat uncovered, carrying gallons (5)
I believe the answer is:
barge
'boat' is the definition.
(I know that barge is a type of boat)
'uncovered carrying gallons' is the wordplay.
'uncovered' becomes 'bare' (similar in meaning).
'carrying' indicates putting letters inside.
'gallons' becomes 'G' (abbreviation).
'bare' placed around 'g' is 'BARGE'.
(Other definitions for barge that I've seen before include "Long boat for freight" , "Narrow boat" , "Canal boat, towed along (5)" , "Possibly lighter" , "Shove - canal boat" .)