Moron insulted diva in opera theatre opening (5)
I believe the answer is:
idiot
'moron' is the definition.
(I know that moron can be written as idiot)
'insulted diva in opera theatre opening' is the wordplay.
'opening' indicates taking the first letters.
The first letters of 'insulted diva in opera theatre' is 'IDIOT'.
(Other definitions for idiot that I've seen before include "eg Dostoevski's Myshkin" , "Ass, nincompoop" , "'The -----'. novel by Dostoyevsky (1868)" , "Foolish character" , "Silly ass" .)