The most common and most likely answer to this crossword clue is the 4 letter word KOAN
K
O
A
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More clues leading to the result KOAN
Buddy explains!

The straight part
We know that "a riddle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a riddle = koan.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 KOAN mean?
KOAN noun- A story about a Zen master and his student, sometimes like a riddle, other times like a fable, which has become an object of Zen study, and which, when meditated upon, may unlock mechanisms in the Zen student’s mind leading to satori.
- A riddle with no solution, used to provoke reflection on the inadequacy of logical reasoning, and to lead to enlightenment.