Seafood swindle requires private investigator (6)
I believe the answer is:
scampi
'seafood' is the definition.
(I've seen this before)
'swindle requires private investigator' is the wordplay.
'swindle' becomes 'scam' (scam is a kind of swindle).
'requires' says to put letters next to each other.
'private investigator' becomes 'pi' ('PI' can be a synonym of 'private investigator').
'scam'+'pi'='SCAMPI'
(Other definitions for scampi that I've seen before include "Crumbed prawns" , "Breaded prawns" , "sort of seafood" , "Norway lobsters -- Dublin Bay prawns" , "Fried seafood" .)