Gangster's partners importing American-caught oysters, say (8)
I believe the answer is:
molluscs
'oysters say' is the definition.
(oyster is a kind of mollusc)
'gangster's partners importing american-caught' is the wordplay.
'gangster's partners' becomes 'molls' (moll can mean a gangster's female partner).
'importing' indicates putting letters inside.
'american' becomes 'us' (abbreviation for United States).
'caught' becomes 'c' (cricket abbreviation).
'us'+'c'='usc'
'molls' going around 'usc' is 'MOLLUSCS'.
(Other definitions for molluscs that I've seen before include "Invertebrates in shells" , "Shellfish" , "Invertebrates such as mussels and snails" , "Cull moss to find snails, mussels, oysters etc" , "Aquatic invertebrates" .)