Historic title of the governor of Mainz? (5)

I believe the answer is:

nizam

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'historic title of the governor' is the definition.
I can't judge whether this defines the answer.

'of mainz?' is the wordplay.
'of' is an anagram indicator (the answer comes from or \'of\' the letters).
'mainz' anagrammed gives 'NIZAM'.

Can you help me to learn more?

(Other definitions for nizam that I've seen before include "Indian son of 20 [SOVRAN]" , "Former ruler of Hyderabad" , "Old Indian prince" .)

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