Large wine container
'large wine container' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for vat that I've seen before include "Large tank or vessel" , "possible source of liquid" , "subject of 17, 8 [CHANCELLORS STATEMENT]" , "Liquid storage receptacle" , "Large vessel for storing liquids and a large tax" .)
(Other definitions for flagon that I've seen before include "Red or white container" , "Large drinks container" , "Large wine vessel" , "Drinking vessel" , "Large container for serving drink" .)