A hotline out of order in comic opera (8)
I believe the answer is:
iolanthe
'comic opera' is the definition.
(Gilbert and Sullivan opera)
'a hotline out of order in' is the wordplay.
'out of order' indicates an anagram.
'in' means one lot of letters goes inside another.
'hotline' with letters rearranged gives 'iolnthe'.
'a' inserted into 'iolnthe' is 'IOLANTHE'.
(Other definitions for iolanthe that I've seen before include "G & S' alternative for "The Peer and the Peri"" , "show" , "G&S operetta" , "one of 63 [OPERETTAS]" , "G&S's eponymous fairy" .)