Mario Puzo book (6)
I believe the answer is:
omerta
'mario puzo book' 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 pledge of silence" , "Mafia code of silence; at Rome (anag.)" , "Mafia code of silence, never inform" , "vow of silence" , "conspiracy of silence" .)