As a system of counting, doctor aided locum (10)

I believe the answer is:

duodecimal

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'as a system of counting' is the definition.
I can't judge whether this defines the answer.

'doctor aided locum' is the wordplay.
'doctor' is an anagram indicator.
'aided'+'locum'='aidedlocum'
'aidedlocum' anagrammed gives 'DUODECIMAL'.

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I've seen this clue in The Guardian.

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