Novel by Mario Puzo published posthumously in 2000 (6)

I believe the answer is:

omerta

(Other definitions for omerta that I've seen before include "Mafia code of silence; at Rome (anag.)" , "conspiracy of silence" , "Mafia code of silence, never inform" , "vow of silence" , "Mafia pledge of silence" .)

I've seen this clue in The Mirror.

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