Big tank (3)
I believe the answer is:
vat
'big tank' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vat that I've seen before include "possible source of liquid" , "Large tank; purchase tax" , "Barrel - tax" , "Large tank for liquids, or kind of tax" , "Tank for holding liquids" .)