A supporter of mine (3-4)

I think it could be either:

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3-4

pit-prop

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'a supporter of mine' is the definition.
(I've seen this before)

'supporter of mine' is the wordplay.
I cannot really understand how this works, but
'supporter' could be 'pro' (I've seen this in another clue) and 'pro' is found within the answer.
'mine' could be 'pit' (down the pit) and 'pit' is found within the answer.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

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