Humiliating failure concealed by Mafia's controller (6)
I believe the answer is:
fiasco
'humiliating failure' is the definition.
(I've seen this before)
'concealed by mafia's controller' is the wordplay.
'concealed' indicates the answer is hidden within the clue.
'FIASCO' is hidden in the letters of 'by mafias controller'.
(Other definitions for fiasco that I've seen before include "Farcical disaster" , "bomb" , "Complete disaster" , "An absolute mess, what a farce" , "Debacle, a right mess" .)