Supporter abbreviated part of speech by old Roman Republican (9)

I believe the answer is:

advocator

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'supporter abbreviated part' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'speech by old roman republican' is the wordplay.
'speech by' becomes 'adv' (I can't explain this - if you can you should give a lot more credence to this answer).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roman' becomes 'cato' (I've seen this before).
'republican' becomes 'R' (abbreviation in US politics).
'adv'+'o'+'cato'+'r'='ADVOCATOR'

'of' is the link.

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I've seen this clue in The Times.

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