The most common and most likely answer to this crossword clue is the 6 letter word TOTHER
I've seen this crossword clue in Financial Times
T
O
T
H
E
R
Buddy explains!
The straight part
We know that "Not this one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not this one = tother.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 TOTHER mean?
TOTHER adjective- The other (of two).
- Other, all others.
- Other. Most often used after the.