Tradition that mum’s kept in the family? (6)

I believe the answer is:

omerta

If this is the answer I cannot explain how the clue works.

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

I've seen this clue in The Times.

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