Tank is huge, not small (3)
I believe the answer is:
vat
'tank' is the definition.
(thesaurus)
'huge not small' is the wordplay.
'huge' becomes 'vast' (I've seen this before).
'not' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vast' with 's' removed is 'VAT'.
'is' acts as a link.
(Other definitions for vat that I've seen before include "Large barrel" , "Container for liquids" , "possible source of liquid" , "Large tank; purchase tax" , "Barrel - tax" .)