The guys went after a copper with ingenuity (6)

I believe the answer is:

acumen

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'ingenuity' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for acumen that I've seen before include "Ability to judge and decide quickly" , "Penetrating insight" , "Forming judgement" , "Keen perception" , "Intellectual sharpness as in business" .)

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

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