Overly tough, extremely firm doctor's fine implement (9)

I believe the answer is:

toothcomb

'fine implement' is the definition.
(comb with fine teeth)

'overly tough extremely firm doctor's' is the wordplay.
'overly' becomes 'too' (synonyms).
'extremely' means to remove the middle letters (extreme, outside letters only).
'firm doctor' becomes 'comb' (I can't explain this - if you can you should give a lot more credence to this answer).
'tough' with its centre taken out is 'th'.
'too'+'th'+'comb'='TOOTHCOMB'

I've seen this clue in The Independent.

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