What could be wrong as a rule in a concerning Roman emperor (8)

I believe the answer is:

aurelian

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

'roman emperor' is the definition.
The definition suggests a singular noun which matches the answer.

'what could be wrong as a rule in a' is the wordplay.
'a rule in a' can be anagrammed to 'AURELIAN'.
But, I am not sure how the anagram is indicated.

'concerning' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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