The incongruity of sarcasm (5)
I believe the answer is:
irony
I believe this clue is a double definition.
'the incongruity' is the first definition.
(irony is a kind of incongruity)
'sarcasm' is the second definition.
(synonyms)
'of' acts as a link.
(Other definitions for irony that I've seen before include "Satirical speech form" , "What "as clear as mud" demonstrates" , "Sarcastic humour" , "style of humour" , "like a metallic element" .)