The most common and most likely answer to this crossword clue is the 9 letter word EFFECTIVE
I've seen this crossword clue in The Times
E
F
F
E
C
T
I
V
E
More clues leading to the result EFFECTIVE
Buddy explains!

The straight part
We know that "Telling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case telling = effective.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 EFFECTIVE mean?
EFFECTIVE noun- A soldier fit for duty
- Having the power to produce a required effect or effects.
- Producing a decided or decisive effect.
- Efficient, serviceable, or operative, available for useful work.
- Actually in effect.
- (of a cycle or divisor) Having no negative coefficients.
- (for any effective theory) approximate; Not describing the fundamental dynamic changes in some system as they happen.