The most common and most likely answer to this crossword clue is the 9 letter word REVEALING
R
E
V
E
A
L
I
N
G
More clues leading to the result REVEALING
Buddy explains!

The straight part
We know that "telltale" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case telltale = revealing.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 REVEALING mean?
REVEALING verb- To uncover; to show and display that which was hidden.
- To communicate that which could not be known or discovered without divine or supernatural instruction.
- Something revealed; a revelation.
- Informative.
- Of clothing: allowing more than is usual to be seen.