Barrel is enormous, not small (3)
I believe the answer is:
vat
'barrel' is the definition.
(a vat is a large tub or barrel)
'enormous not small' is the wordplay.
'enormous' becomes 'vast' (I've seen this before).
'not' is a deletion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vast' with 's' taken out is 'VAT'.
'is' is the link.
(Other definitions for vat that I've seen before include "Large tank or vessel" , "Container for liquids" , "subject of 17, 8 [CHANCELLORS STATEMENT]" , "Large tank; purchase tax" , "possible source of liquid" .)