Magistrates taking native American families around UN (8)

I believe the answer is:

tribunes

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'magistrates' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.

'native american families around un' is the wordplay.
'native american families' becomes 'tribes' (I can't explain this - if you can you should believe this answer much more).
'around' means one lot of letters goes inside another.
'tribes' placed around 'un' is 'TRIBUNES'.

'taking' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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