High-ranking Mafioso has enemy leader polished off (4)

I believe the answer is:

done

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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'.
'don'+'e'='DONE'

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(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".)

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