Sardonic wit (5)
I believe the answer is:
irony
'sardonic wit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for irony that I've seen before include "Socrates used it" , "Kind of sarcasm, sounds metallic" , "Use of words to convey the opposite of what they normally mean" , "Mild sarcasm" , "What "as clear as mud" demonstrates" .)