Seafood racket's very good (6)
I believe the answer is:
scampi
'seafood' is the definition.
(type of seafood)
'racket's very good' is the wordplay.
'racket' becomes 'scam' (racket is a type of scam or scheme).
'very good' becomes 'pi' (old-fashioned term meaning pious).
'scam'+'pi'='SCAMPI'
(Other definitions for scampi that I've seen before include "Large prawns as food" , "Edible crustaceans" , "Shellfish" , "Large sauteed prawns" , "Prawn dish" .)