Cask (6)
'cask' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for barrel that I've seen before include "VAT?" , "Round wooden vessel." , "Large round container as for beer" , "Cheerful old song: 'Roll out the . . . . . .'" , "Beer cask" .)
(Other definitions for vat that I've seen before include "Liquid storage receptacle" , "Large tank for liquids, or kind of tax" , "Barrel - tax" , "Fermentation vessel" , "Deep tank or tax duty" .)
'cask' is the definition.
(I know that tun is a type of cask)
This is all the clue.
(Other definitions for tun that I've seen before include "butt" , "vessel" , "'Large cask, for beer perhaps (3)'" , "Large cask for wine or beer" , "beer barrel" .)