The most common and most likely answer to this crossword clue is the 11 letter word EXPOSTULATE
I've seen this crossword clue in The Guardian
E
X
P
O
S
T
U
L
A
T
E
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 = expostulate.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 EXPOSTULATE mean?
EXPOSTULATE verb- To protest or remonstrate; to reason earnestly with a person on some impropriety of conduct.