Cosa Nostra (5)
I believe the answer is:
mafia
'cosa nostra' is the definition.
(Cosa Nostra is an Italian term for the mafia)
This is the entire clue.
(Other definitions for mafia that I've seen before include "organised criminals" , "International crime organisation originating in Sicily" , "Criminal organisation also known as Cosa Nostra" , "Gangsters" , "Sicilian secret society" .)