A lion disrupted the operetta (8)
I believe the answer is:
iolanthe
'operetta' is the definition.
(I know that Iolanthe is a operetta)
'a lion disrupted the' is the wordplay.
'disrupted' indicates an anagram.
'a'+'lion'='alion'
'alion' anagrammed gives 'iolan'.
'iolan'+'the'='IOLANTHE'
(Other definitions for iolanthe that I've seen before include "G&S piece" , "G & S' alternative for "The Peer and the Peri"" , "show" , "Staged work" , "G&S operetta" .)