Nasty smile (5)
I believe the answer is:
sneer
'nasty smile' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sneer that I've seen before include "Scoff, scorn" , "Be derisive; Sheridan's Critic" , "Derisive remark" , "Show contempt with scornful expression or words" , "Speak contemptuously" .)