Epicurean Mafia boss involved in robbery (8)
I believe the answer is:
hedonist
'epicurean' is the definition.
(I've seen this before)
'mafia boss involved in robbery' is the wordplay.
'mafia boss' becomes 'don' (I've seen this before).
'involved in' means one lot of letters goes inside another.
'robbery' becomes 'heist' (heist is a kind of robbery).
'don' placed within 'heist' is 'HEDONIST'.
(Other definitions for hedonist that I've seen before include "he puts pleasure before work" , "A seeker after luxury" , "bon vivant" , "sybarite" , "He believes pleasure is the chief good in life" .)