Musical dramas (6)
'musical dramas' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for operas that I've seen before include "La Traviata, Aida" , "Dramatic works set to music" , "They're written by 1 across" , "Cos? fan tutte and The Magic Flute" , "Stage dramas" .)
(Other definitions for operettas that I've seen before include "10d [IOLANTHE], 45d [RUDDIGORE] and 6a [PRINCESS IDA], say" , "Light musicals for poetaster" , "Musical compositions as by Gilbert & Sullivan" , "Performances" .)