. . . a pal once converted by a Mafioso (2,6)
I believe the answer is:
'a mafioso' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'a pal once converted' is the wordplay.
'converted' indicates an anagram.
'palonce' with letters rearranged gives 'lcapone'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for al capone that I've seen before include "Notorious Chicago gangster nicknamed Scarface" , "Notorious Chicago gangster arrested for tax evasion" , "US gangster in Chicago, arrested for tax evasion" , "Notorious gangster, d. 1947" , "Chicago mobster jailed for tax evasion" .)