The most common and most likely answer to this crossword clue is the 9 letter word COGNITION
C
O
G
N
I
T
I
O
N
Buddy explains!

The straight part
We know that "Reason" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reason = cognition.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 COGNITION mean?
COGNITION noun- The process of knowing, of acquiring knowledge and understanding through thought and through the senses.
- A result of a cognitive process.