What may track down renegade changing sides in fight? (8)

I believe the answer is:

detector

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'fight?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for detector that I've seen before include "Device warning of, eg, smoke" , "Electronic device" , "Device for monitoring hazards" , "Sensor" .)

I've seen this clue in The Times.

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