Revealing Rossini's libretto? (8)

I believe the answer is:

telltale

I'm a little stuck... Click here to teach me more about this clue!

'revealing' is the definition.
(I know that revealing can be written as telltale)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for telltale that I've seen before include "Revelatory - blabbermouth" , "Betraying" , "Person who blabs, or something revealing" , "Outward indication of something concealed" , "One revealing secrets" .)

I've seen this clue in The Globe and Mail.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.