Criminal organisation (3)
'criminal organisation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mafia that I've seen before include "Sicilian one" , "Criminal organisation with origin in Sicily" , "Sicilian criminal organisation" , "Gangsters" , "Criminal organisation also known as Cosa Nostra" .)
(Other definitions for mob that I've seen before include "Crowd round" , "Mafiosi" , "Horde" , "Ugly crowd" , "Rowdy lot" .)