A nurse working on insanity (8)
I believe the answer is:
unreason
'insanity' is the definition.
(unreason is a kind of insanity)
'a nurse working on' is the wordplay.
'working' indicates anagramming the letters.
'a'+'nurse'='anurse'
'anurse' with letters rearranged gives 'unreas'.
'unreas'+'on'='UNREASON'
(Other definitions for unreason that I've seen before include "The absence of rationality or logic" , "foolishness" , "Irrationality" , "nonsense" , "insanity" .)