Workers keep score using brainpower (8)

I believe the answer is:

mentally

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'workers keep score using brainpower' is the definition.
I can't tell whether this definition defines the answer.

'workers keep score' is the wordplay.
'workers' becomes 'men' (men can mean staff or employees).
'keep score' becomes 'tally' (synonyms).
'men'+'tally'='MENTALLY'

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(Other definitions for mentally that I've seen before include "not physically" , "in the head" , "Intellectually" , "by way of intellect" , "in their heads" .)

I've seen this clue in The Telegraph.

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