The most common and most likely answer to this crossword clue is the 3 plus 3 letter word RUN OFF
R
U
N
O
F
F
Buddy explains!
The straight part
We know that "Decisive event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decisive event = run off.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 RUN OFF mean?
RUN OFF noun- A run-off area; a paved area around the track at a spot that drivers are likely to go off, as opposed to a gravel trap, grass or a barrier.
- That portion of precipitation or irrigation on an area which does not infiltrate or evaporate, but instead is discharged from the area.
- Dissolved chemicals, etc, included in such water.
- A second or further round of an indecisive election, after other candidates (often all but the last two) have been eliminated.