Quickly produce winning blow (8)
I believe the answer is:
knockout
'winning blow' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for knockout that I've seen before include "Stun - defeat - destroy" , "Decisive punch" , "triumph" , "End of bout" , "Result of last punch" .)