False earnestness (5)
I believe the answer is:
smarm
'false earnestness' 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 "Superficially ingratiating behaviour" , "Obsequious flattery" , "Fawning" , "Smooth talking" , "oil" .)