Villain in the opera "Tosca" (3)
I believe the answer is:
rat
'villain' is the definition.
(thesaurus)
'in the opera tosca' is the wordplay.
'in' indicates a hidden word.
'RAT' can be found hidden inside 'the opera tosca'.
(Other definitions for rat that I've seen before include "Plague-carrying rodent" , "Unpleasant character" , "One deserting sinking ship" , "Desert - rodent" , "Domestic rodent" .)