Mafioso's code of silence
I believe the answer is:
'mafioso's 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; at Rome (anag.)" , "Mafia pledge of silence" , "Mafia code of silence, never inform" , "conspiracy of silence" , "vow of silence" .)