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