The --, Gilbert and Sullivan opera (6)
I believe the answer is:
mikado
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">the gilbert and sullivan opera</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mikado that I've seen before include "Japanese ruler" , "one of 63 [OPERETTAS]" , "Japanese emperor, operetta and delicious biscuit" , "Gilbert and Sullivan opera" , "Gilbert and Sullivan's emperor" .)