Villain of operas Callas appeared in (6)
I believe the answer is:
rascal
'villain' is the definition.
(I know that rascal is a type of villain)
'operas callas appeared in' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RASCAL' is hidden within 'operas callas appeared'.
'of' acts as a link.
(Other definitions for rascal that I've seen before include "Rogue or cheeky child" , "Rogue, scamp" , "naughty child" , "Mischievous person or confused lascar" , "wretch" .)