Mordant (8)
I believe the answer is:
sardonic
'mordant' is the definition.
(synonyms)
This is all the clue.
(Other definitions for sardonic that I've seen before include "Scornfully mocking" , "Tart" , "Mocking or scornful" , "Dry, cynically mocking" , "Disdainfully or ironically humorous" .)