Yet such talkers may not be on the level! (6)
I believe the answer is:
smooth
'the level' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for smooth that I've seen before include "Level; suave" , "Free from roughness in appearance or sound" , "Free from roughness or irregularities" , "Suavely charming" , "Without lumps" .)