Having supreme authority appoint entirely new order (15)

I believe the answer is:

plenipotentiary

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

'having supreme authority' is the definition.
I can't tell whether this definition defines the answer.

'appoint entirely new order' is the wordplay.
'new order' indicates anagramming the letters.
'appoint'+'entirely'='appointentirely'
'appointentirely' anagrammed gives 'PLENIPOTENTIARY'.

Can you help me to learn more?

(Other definitions for plenipotentiary that I've seen before include "Powerful diplomat" , ""The Ambassador"" .)

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

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