Aristos expert with their fists? (9)

I believe the answer is:

archdukes

I'm a little stuck... Click here to teach me more about this clue!

'aristos expert' is the definition.
I can't tell whether this defines the answer.

'with their fists?' is the wordplay.
'with' is a charade indicator (letters next to each other).
'their' becomes 'arch' (I can't justify this - if you can you should believe this answer much more).
'fists?' becomes 'dukes' (I've seen this before).
'arch' next to 'dukes' is 'ARCHDUKES'.

Can you help me to learn more?

I've seen this clue in The Guardian.

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