Thus employed, make maximum profit (9)

I believe the answer is:

gainfully

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'thus employed make maximum profit' is the definition.
I can't tell whether this defines the answer.

'make maximum profit' is the wordplay.
'make' becomes 'gain' ('gain' can be a synonym of 'make').
'maximum profit' becomes 'fully' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gain'+'fully'='GAINFULLY'

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I've seen this clue in the Evening Standard.

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