The most common and most likely answer to this crossword clue is the 11 letter word DETERMINANT
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More clues leading to the result DETERMINANT
Buddy explains!

The straight part
We know that "decisive factor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decisive factor = determinant.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 DETERMINANT mean?
DETERMINANT noun- A determining factor; an element that determines the nature of something.
- A scalar that encodes certain characteristics of a given transformation matrix; the unique scalar function over square matrices which is distributive over matrix multiplication, multilinear in the rows and columns, and takes the value 1 for the unit matrix; abbreviated as: det.
- A substance that causes a cell to adopt a particular fate.
- Serving to determine or limit; determinative.