The most common and most likely answer to this crossword clue is the 9 plus 3 letter word CERTAINLY NOT
I've seen this crossword clue in The Guardian
C
E
R
T
A
I
N
L
Y
N
O
T
More clues leading to the result CERTAINLY NOT
Buddy explains!
The straight part
We know that "No" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case no = certainly not.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.