Code of silence (6)
I believe the answer is:
omerta
'code of silence' is the definition.
(code of silence in the Mafia)
This is the entire clue.
(Other definitions for omerta that I've seen before include "vow of silence" , "Mafia pledge of silence" , "Mafia code of silence, never inform" , "Mafia code of silence; at Rome (anag.)" , "conspiracy of silence" .)