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 subsidiary indication.
'high-ranking mafioso' becomes 'don' (a don is a high-ranking mafioso).
'has' is a charade indicator (letters next to each other).
'leader' indicates taking the first letters.
The initial letter of 'enemy' is 'e'.
(Other definitions for done that I've seen before include "Swindled", "Cooked; carried out", "Achieved", "Finished or ended", "Thoroughly cooked; accomplished", "See 23", "Finished like node", "Artist's workroom", "Performed" and "(Of food) well cooked".)