Sneering (5)
I believe the answer is:
snide
'sneering' is the definition.
(I know that sneering can be written as snide)
This is all the clue.
(Other definitions for snide that I've seen before include "Derogatory in an insinuating way" , "Unpleasantly mocking" , "Delivering cuts" , "Dines (anag.)" , "Sarcastic" .)