Silver-tongued (4)
'silver-tongued' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for glib that I've seen before include "slick in speech" , "Plausibly fluent" , "smooth" , "Fluent but evasive" , "Having only superficial plausibility" .)
(Other definitions for persuasive that I've seen before include "Providing sound reasoning" , "Convincing" .)