Irrationality, madness (8)
I believe the answer is:
unreason
This clue is a double definition.
'irrationality' is the first definition.
(I know that irrationality can be written as unreason)
'madness' is the second definition.
(I know that unreason is a type of madness)
(Other definitions for unreason that I've seen before include "nonsense" , "The absence of rationality or logic" , "insanity" , "Irrationality" , "foolishness" .)