Mocking smile (5)
I believe the answer is:
sneer
'mocking smile' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sneer that I've seen before include "A scornful expression or remark" , "Curl one's lip" , "Contemptuous gesture" , "Show contempt with scornful expression or words" , "Smile contemptuously" .)