The most common and most likely answer to this crossword clue is the 9 letter word RATIONALE
I've seen this crossword clue in The Times
R
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More clues leading to the result RATIONALE
- … A justification to share out beer?
- Intelligent European principle
- Explanation of desert island having left half missing
- What some TT reformers might do for an explanation
- Be careful with drink — that's the thinking!
- Limit beer in grounds
- Restrict supply of booze? There’s a reason
- Reason to limit the supply of alcohol
- Reason gets one into a real mess
- Good reason to limit the supply of alcohol
Buddy explains!

The straight part
We know that "Good reason" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good reason = rationale.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 RATIONALE mean?
RATIONALE noun- An explanation of the basis or fundamental reasons for something.
- A justification or rationalization for something.
- A liturgical vestment worn by some Christian bishops of various denominations.