Author's not upset by article's reason for insanity? (7)
I believe the answer is:
'reason for insanity?' is the definition.
(reason is an opposite or antonym of insanity)
'author's not upset by article's' is the wordplay.
'author's' becomes 'my' (belonging to the person writing).
'upset' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'article' becomes 'an' (indefinite article).
'mynot' in reverse letter order is 'tonym'.
'tonym' put after 'an' is 'ANTONYM'.
(Other definitions for antonym that I've seen before include "opposing description" , "A word's opposite" , "As 'happy' is to 'sad' or 'long' to 'short'" , "that means reverse" , ""Loud" as opposed to "quiet", for example" .)