Swindle over very good seafood (6)
I believe the answer is:
scampi
'seafood' is the definition.
(type of seafood)
'swindle over very good' is the wordplay.
'swindle' becomes 'scam' (scam is a kind of swindle).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'very good' becomes 'pi' (old-fashioned slang for pious).
'scam'+'pi'='SCAMPI'
(Other definitions for scampi that I've seen before include "sort of seafood" , "Crumbed prawns" , "Shellfish" , "Prawns, usually fried" , "Fried seafood" .)