Unrepeatable action on foe returning open fire (3-3)

I believe the answer is:

one-off

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But I can't explain it at all! Could you help me understand?

(Other definitions for one-off that I've seen before include "Unique person or occurrence" , "Not repeated, being a single event or object" , "Something unique" , "A single event, not reproduced or repeated" , "Happening only once" .)

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

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