Unctuous, glib, and that's flat! (6)
I believe the answer is:
smooth
I believe this clue is a triple definition!
'unctuous' is the first definition.
(thesaurus)
'glib' is the second definition.
(I've seen this in another clue)
'and' is a link.
'that's flat' is the third definition.
(thesaurus)
(Other definitions for smooth that I've seen before include "Free from roughness, bumps etc." , "Not rough or wrinkled" , "Unruffled" , "Calm" , "Flat - consistent - plausible" .)