The most common and most likely answer to this crossword clue is the 6 letter word SANITY
I've seen this crossword clue in The Times
S
A
N
I
T
Y
More clues leading to the result SANITY
- Speak about idiot making sense
- Reason for one to eat egg
- State in which fool's seen sense
- Possessing reason for one to accommodate fool
- After treatment, stay in normal frame of mind
- Holy state banishes court for good reason
- Reason to speak about Charlie
- Reason, perhaps, egg breaks
- Godliness has no heart or presence of mind?
- Reason, for example, money’s held back therein
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 = sanity.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 SANITY mean?
SANITY noun- The condition of being sane.
- Reasonable and rational behaviour.