Sarcastic or paradoxical (6)
I believe the answer is:
ironic
I believe this clue is a double definition.
'sarcastic' is the first definition.
(similar in meaning)
'paradoxical' is the second definition.
(I've seen this before)
(Other definitions for ironic that I've seen before include "Describes a statement that says the opposite of what is clearly the case" , "Suggesting the opposite for humorous effect" , "Wry, sarcastic" , "Mockingly humorous" , "Characterised by mildly sardonic wit" .)