Insincerely fluent (4)
I believe the answer is:
glib
'insincerely fluent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for glib that I've seen before include "Shallowly articulate" , "Superficially facile with words" , "Casually fluent" , "Fluent and plausible" , "Plausibly fluent" .)