Inverted, the box containing a tank (3)
I believe the answer is:
vat
'tank' is the definition.
(thesaurus)
'inverted the box containing a' is the wordplay.
'inverted' is a reversal indicator.
'the box' becomes 'tv'.
'containing' indicates putting letters inside.
'tv' going around 'a' is 'tav'.
'tav' written backwards gives 'VAT'.
(Other definitions for vat that I've seen before include "Large tank or vessel" , "Tank for holding liquids" , "Large tub; tax" , "Large container for liquid" , "Large vessel for storing liquids and a large tax" .)