The most common and most likely answer to this crossword clue is the 1 letter word J
I've seen this crossword clue in The Sunday Telegraph
J
More clues leading to the result J
- For an audience, a patient man records an average one
- 2 x ASSes plus 1 x IN.
- J
- Joiner from A Midsummer Night’s Dream, initially not primarily mechanical
- 20 INJURE/EMMETS
- Janitor at first reluctant to ring about item of cleaning equipment
- Not without reason, only partly believes an expert or trusts a nerdSANE – A lovely double lurker to finish – found partly in believeS AN Expert or trustS A NErd
- was our favourite because of the chuckle it gave us.
- The Messiah at first reluctant to engage cleaner
- Cleaner unwilling to be subjugated by Lord casually
Buddy explains!

The straight part
We know that "Not without reason" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not without reason = j.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 J mean?
J noun- A term for a marijuana cigarette ('joint').
- An alternative version of i, the positive square root of -1; used in the context of electronics.
- The second unit vector, after i