Container for liquid (6)
'container for liquid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bottle that I've seen before include "It keeps drink" , "Glass container - firmness of resolve" , "Device used by beer drinkers" , "Nebuchadnezzar?" , "Glass or plastic container" .)
(Other definitions for vat that I've seen before include "Deep tank or tax duty" , "Large tank for liquids, or kind of tax" , "Container for liquids" , "Tank for holding liquids" , "Liquid storage receptacle" .)