A vessel as opposed to a tin opener? (3)
I believe the answer is:
vat
'a vessel' is the definition.
(vat is a kind of vessel)
'opposed to a tin opener?' is the wordplay.
'opposed to' becomes 'v' (abbreviation for versus).
'opener?' indicates taking the first letters (I've seen 'opener' mean this).
The initial letter of 'tin' is 't'.
'v'+'a'+'t'='VAT'
'as' acts as a link.
(Other definitions for vat that I've seen before include "Large tank or vessel" , "Large tank; purchase tax" , "Large tub; tax" , "Tank for holding liquids" , "Large barrel" .)