Ingratiating (6)
I believe the answer is:
smarmy
'ingratiating' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for smarmy that I've seen before include "Obsequious" , "Smooth" , "Bootlicking" , "Unpleasantly suave or ingratiating" , "Unctuous" .)