Underworld society (5)
I believe the answer is:
mafia
'underworld society' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mafia that I've seen before include "connection claimed to 23 [JFK]" , "Secret criminal organisation" , "Sicilian criminal organisation" , "Gangsters" , "Group exerting a hidden sinister influence" .)