Large tub and tax (3)
I believe the answer is:
vat
'large tub and tax' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for vat that I've seen before include "Large tub; tax" , "Deep tank or tax duty" , "Liquid storage receptacle" , "Tank for holding liquids" , "possible source of liquid" .)