Yes, MAFFIA is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, F 4, F 4, I 1, A 1.

Is it a valid word?

MAFFIA is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a crime syndicate in the United States; organized in families; believed to have important relations to the Sicilian Mafia
a secret terrorist group in Sicily; originally opposed tyranny but evolved into a criminal organization in the middle of the 19th century
any tightly knit group of trusted associates

Also: cosa nostra, sicilian mafia.