Stupidity about son's mental disorder (8)
I believe the answer is:
insanity
'mental disorder' is the definition.
(thesaurus)
'stupidity about son's' is the wordplay.
'stupidity' becomes 'inanity' (I've seen this before).
'about' is an insertion indicator.
'inanity' enclosing 's' is 'INSANITY'.
(Other definitions for insanity that I've seen before include "mania" , "Mental derangement" , "Lunacy, derangement" , "Lunacy, madness" , "folly" .)