Boat is exposed, shipping gallons (5)
I believe the answer is:
barge
'boat' is the definition.
(barge is a kind of boat)
'exposed shipping gallons' is the wordplay.
'exposed' becomes 'bare' (I've seen this in other clues).
'shipping' means one lot of letters goes inside another (inserted letters are carried or shipped).
'gallons' becomes 'g' (abbreviation).
'bare' going around 'g' is 'BARGE'.
'is' is the link.
(Other definitions for barge that I've seen before include "Possibly lighter" , "Shove - canal boat" , "Canal-boat" , "Narrow boat" , "Flat-bottom boat" .)