The most common and most likely answer to this crossword clue is the 9 letter word SINCERITY
S
I
N
C
E
R
I
T
Y
More clues leading to the result SINCERITY
Buddy explains!

The straight part
We know that "truth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case truth = sincerity.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 SINCERITY mean?
SINCERITY noun- The quality or state of being sincere; honesty of mind or intention; freedom from simulation, hypocrisy, disguise, or false pretense.