As an operatic part, a villain (3)
I believe the answer is:
rat
'a villain' is the definition.
(thesaurus)
'as an operatic part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'RAT' is hidden within 'as operatic'.
(Other definitions for rat that I've seen before include "Treacherous person" , "one who betrayed" , "one who deserts the sinking ship?" , "Destructive rodent" , "Unpleasant character" .)