The most common and most likely answer to this crossword clue is the 4 letter word KAYO
I've seen this crossword clue in The Times
K
A
Y
O
More clues leading to the result KAYO
- Old bovine knocked over, end of contest
- A decisive blow, all right, with round coming to the end
- Decisive blow spinning ruminant round
- Floor is fine for cycling
- Punched by right hand of Dempsey, strong thing felled — end of fight!
- Fight’s ending, as old bovine turned over
- End of bout nothing to talk about
- Keep talking about old floor
- Box to deliver this – it’s a winner
- After switching sections, approve floor
Buddy explains!

The straight part
We know that "A decisive blow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a decisive blow = kayo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does KAYO mean?
KAYO noun- A knockout (in boxing).
- To knock someone out, or render them unconscious or senseless.