The most common and most likely answer to this crossword clue is the 9 letter word FICTIONAL
I've seen this crossword clue in The Guardian
F
I
C
T
I
O
N
A
L
More clues leading to the result FICTIONAL
Buddy explains!

The straight part
We know that "It’s not true" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s not true = fictional.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 FICTIONAL mean?
FICTIONAL adjective- Invented, as opposed to real.
- Containing invented elements.