Wide container (3)
(Other definitions for vat that I've seen before include "Large barrel" , "Large container for liquid" , "subject of 17, 8 [CHANCELLORS STATEMENT]" , "Large tank; purchase tax" , "Barrel - tax" .)
'wide container' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tub that I've seen before include "Container, usually for washing" , "Bath; leaky ship" , "Low wide container" , "Plastic container" , "Barrel; boat" .)