Style of humour (5)
I believe the answer is:
irony
'style of humour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for irony that I've seen before include "Satire, sarcasm" , "Figure of speech" , "Kind of sarcasm, sounds metallic" , "Use of words to convey the opposite of what they normally mean" , "Gentle sarcasm" .)