Smile contemptuously (5)
I believe the answer is:
sneer
'smile contemptuously' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sneer that I've seen before include "Contemptuous look" , "Contemptuous or mocking smile" , "Curl one's lip" , "Contemptuous expression" , "Speak contemptuously" .)