Sequel to Puzo's "The Last Don" (6)
I believe the answer is:
omerta
'sequel to puzo's the last don' 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 "vow of silence" , "Mafia pledge of silence" , "conspiracy of silence" , "Mafia code of silence, never inform" , "Mafia code of silence; at Rome (anag.)" .)