This person's overturned umpire's rule (8)

I believe the answer is:

imperium

'rule' is the definition.
(thesaurus)

'this person's overturned umpire's' is the wordplay.
'this person's' becomes 'I'm' (('I am' - 'this person' means the crossword setter).
'overturned' is an anagram indicator.
'umpire' anagrammed gives 'perium'.
'im'+'perium'='IMPERIUM'

I've seen this clue in The Guardian.

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