Fluent and plausible (4)
I believe the answer is:
glib
'fluent and plausible' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for glib that I've seen before include "Fluent but evasive" , "Of speech, facile but insincere" , "Slippery in speech" , "Fluent but insincere" , "Having only superficial plausibility" .)