The most common and most likely answer to this crossword clue is the 3 plus 5 letter word LET ALONE
I've seen this crossword clue in Financial Times
L
E
T
A
L
O
N
E
More clues leading to the result LET ALONE
- Never mind what landlords do unaided
- Not to mention bird’s claw found in shelter
- The French story about working much less
- Female coming out ignored much less
- Avoid touching claw stuck in swimmer’s back
- Much less neglected
- Much less vicious, less hard individual
- Never caressed? Never mind!
- Don’t touch without even considering
- Much less left by Greek character, single
Buddy explains!
The straight part
We know that "Not to mentio" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not to mentio = let alone.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.