Long-time heretic, one replacing a surgeon (12)

'surgeon' is the definition.
(veterinary surgeon)

'long-time heretic one replacing a' is the wordplay.
I cannot quite understand how this works, but
'time' could be 't' (abbreviation) and 't' is present in the answer.
'heretic' could be 'arian' (Arianism is a form of heresy) and 'arian' is present in the answer.
'one' could be 'er' (I've seen this in other clues) and 'er' is found within the answer.
The remaining letters 'vein' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...