Villain in opera venue discovered after opening of Rigoletto (6)
I believe the answer is:
rascal
'villain' is the definition.
(I know that rascal is a type of villain)
'opera venue discovered after opening of rigoletto' is the wordplay.
'opera venue' becomes 'la scala' (Milanese opera house).
'discovered' says to take the centre (dis-covered - outer letters removed).
'after' says to put letters next to each other.
'opening of' indicates taking the first letters.
The middle letters of 'lascala' are 'ascal'.
The initial letter of 'rigoletto' is 'r'.
'ascal' after 'r' is 'RASCAL'.
'in' acts as a link.
(Other definitions for rascal that I've seen before include "Mischievous person or confused lascar" , "Reprobate" , "Scallywag" , "Scoundrel or mischievous child" , "Cad" .)