Trial cast upset opera producer (9)
I believe the answer is:
scarlatti
'opera producer' is the definition.
(Domenico Scarlatti, Italian composer of operas)
'trial cast upset' is the wordplay.
'upset' indicates anagramming the letters.
'trial'+'cast'='trialcast'
'trialcast' anagrammed gives 'SCARLATTI'.
(Other definitions for scarlatti that I've seen before include "Cantatas by him?" , "Alessandro and Domenico --, Italian composers" , "Composer - trails cat (anag)" , "scorer" , "opera producer" .)