Smug grin (5)
I believe the answer is:
smirk
'smug grin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smirk that I've seen before include "Complacent smile" , "Leer" , "Smile scornfully" , "Self-satisfied grin" , "Show self-complacent satisfaction" .)