Lip-curling smile (5)
I believe the answer is:
sneer
'lip-curling smile' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sneer that I've seen before include "Show contempt with scornful expression or words" , "Contemptuous expression" , "Speak scornfully" , "Deride, jeer" , "Express cynical contempt" .)