Filling seat, doctor mingles (11)

I believe the answer is:

amalgamates

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'mingles' is the definition.
('amalgamate' can be a synonym of 'mingle')

'filling seat doctor' is the wordplay.
'filling' becomes 'amalgam' (I am not sure about this - if you are sure you should believe this answer much more).
'doctor' is an anagram indicator.
'seat' anagrammed gives 'ates'.
'amalgam'+'ates'='AMALGAMATES'

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

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