Unpleasantly suave (6)
I believe the answer is:
smarmy
'unpleasantly suave' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for smarmy that I've seen before include "Bootlicking" , "Excessively flattering (R, not L)" , "Oily" , "Excessively ingratiating" , "Fawning" .)