Insincere charm (5)
I believe the answer is:
smarm
'insincere charm' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smarm that I've seen before include "oil" , "Superficially ingratiating behaviour" , "Fawning" , "Obsequious flattery" , "Smooth talking" .)