The most common and most likely answer to this crossword clue is the 9 letter word VERIDICAL
I've seen this crossword clue in The Times
V
E
R
I
D
I
C
A
L
Buddy explains!
The straight part
We know that "Truthful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case truthful = veridical.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 VERIDICAL mean?
VERIDICAL adjective- True.
- Pertaining to an experience, perception, or interpretation that accurately represents reality; as opposed to imaginative, unsubstantiated, illusory, or delusory.