Musical (6)
(Other definitions for operas that I've seen before include "Dramas set to music" , "Musical works" , "Dramatic musicals in which words are sung" , "Dramatic works set to music" , "Musical dramas" .)
'musical' is the definition.
(I know that musical can be written as melodic)
This is the entire clue.
(Other definitions for melodic that I've seen before include "Harmonious" , "Old mice (anag.)" , "nice on the ear" , "Pleasant to listen to" , "Of music, tuneful and sweet" .)