The most common and most likely answer to this crossword clue is the 4 plus 3 plus 3 plus 5 letter word GAME SET AND MATCH
G
A
M
E
S
E
T
A
N
D
M
A
T
C
H
More clues leading to the result GAME SET AND MATCH
Buddy explains!
The straight part
We know that "decisive victory" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decisive victory = game set and match.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.