Girl to inform on one country's emperor (9)

I believe the answer is:

vitellius

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'country's emperor' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'girl to inform on one' is the wordplay.
'girl' becomes 'vi' (short for Violet).
'to inform' becomes 'tell' (telling is a kind of informing).
'on one' becomes 'ius' (I can't explain this - if you can you should believe this answer much more).
'vi'+'tell'+'ius'='VITELLIUS'

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

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