Mean grin (5)
I believe the answer is:
'mean grin' 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 "A scornful expression or remark" , "Show cynical contempt" , "Show contempt by contorting the face" , "Contemptuous expression" , "Express cynical contempt" .)