High-ranking Mafioso has enemy leader polished off (4)
I believe the answer is:
done
'polished off' is the definition.
(when one is 'polished off' they are done)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for done that I've seen before include "Managed" , "Performed" , "Robbed" , "Achieved" , "Thoroughly cooked; accomplished" .)