Bon vivant Mafia boss in armed hold-up (8)
I believe the answer is:
hedonist
'bon vivant' is the definition.
(thesaurus)
'mafia boss in armed hold-up' is the wordplay.
'mafia boss' becomes 'don' (I've seen this before).
'in' means one lot of letters goes inside another.
'armed hold-up' becomes 'heist' (I've seen this before).
'don' going inside 'heist' is 'HEDONIST'.
(Other definitions for hedonist that I've seen before include "Epicurean" , "seeker of fun" , "He believes pleasure is the chief good in life" , "Self-indulgent type" , "One who lives only for pleasure" .)