Villain of operas Callas appeared in (6)
I believe the answer is:
rascal
'villain' is the definition.
(rascal is a kind 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' is the 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" .)