Keg (3)
(Other definitions for vat that I've seen before include "Large tank or vessel" , "Large barrel" , "Large container for liquid" , "Large vessel for storing liquids and a large tax" , "Large tank; purchase tax" .)
'keg' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for barrel that I've seen before include "Round wooden cask made by cooper" , "''Roll out the ..., let's have a ... of fun'' (WWII song)" , "Beer measure; gun part" , "Tun" , "Revolving drum" .)