In attack, coppers possibly traded knight for rook (6)

I believe the answer is:

charge

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'in attack' is the definition.
'charge' can be an answer for 'attack' (I've seen this before). I am not sure about the 'in' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for charge that I've seen before include "Accuse of a crime" , "Move quickly and violently" , "Entrust (with a task)" , "Lord Cardigan's order to his brigade" , "Formally accuse; rush" .)

I've seen this clue in The Times and The Sunday Times.

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