The most common and most likely answer to this crossword clue is the 9 letter word TROUNCING
I've seen this crossword clue in The Guardian
T
R
O
U
N
C
I
N
G
More clues leading to the result TROUNCING
Buddy explains!

The straight part
We know that "Decisive defeat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decisive defeat = trouncing.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 TROUNCING mean?
TROUNCING verb- To beat severely; to thrash.
- To beat or overcome thoroughly, to defeat heavily; especially (games) to win against (someone) by a wide margin.
- To chastise or punish physically or verbally; to scold with abusive language.
- To punish by bringing a lawsuit against; to sue.
- To walk heavily or with some difficulty; to tramp, to trudge.
- To pass across or over; to traverse.
- To travel quickly over a long distance.
- Present participle of trounce: a thorough defeat.