Unrepeatable action on foe returning open fire (3-3)
I believe the answer is:
one-off
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" .)