High-ranking Mafioso has enemy leader polished off (4)
I believe the answer is:
Here is my best explanation:
'polished off' is the definition.
(when one is 'polished off' they are done)
'high-ranking mafioso has enemy leader' is the wordplay.
'high-ranking mafioso' becomes 'don' (a don is a high-ranking mafioso).
'has' is a charade indicator (letters next to each other).
'leader' suggests taking the first letters.
The first letter of 'enemy' is 'e'.
(Other definitions for done that I've seen before include "Managed", "that may be said to seal a deal", "Achieved", "This settled", "Agreed", "Thoroughly cooked; accomplished", "Cooked; carried out", "Performed", "Concluded (and dusted?)" and "Completed".)