Jong Un rejected a party for Japanese leader (6)

I believe the answer is:

mikado

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'leader' is the definition.
(I have seen 'national leader' mean 'mikado' so perhaps 'leader' could also mean 'mikado')

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mikado that I've seen before include "Japanese 2" , "Japanese emperor's title" , "Gilbert and Sullivan opera" , "G&S emperor" , "mperor" .)

I've seen this clue in the Sydney Morning Herald.

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