A Savoy opera (8)
(Other definitions for iolanthe that I've seen before include "G&S twenty three down" , "The Peer and the Peri is its alternative title" , "show" , "G&S operetta" , "G&S's eponymous fairy" .)
(Other definitions for ruddigore that I've seen before include "Gilbert and Sullivan opera" , "G & S' alternative for "The Witch''s Curse"" , "One of 63 [OPERETTAS]" , "G&S operetta" , "show" .)