Gilbert and Sullivan opera (8)
I believe the answer is:
iolanthe
'gilbert and sullivan opera' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iolanthe that I've seen before include "G&S operetta" , "G&S twenty three down" , "G&S's eponymous fairy" , "The Peer and the Peri is its alternative title" , "Gilbert and Sullivan opera" .)