Novel, revolutionary trouble for old ruler (6)
I believe the answer is:
mikado
'for old ruler' is the definition.
(historical title of the emperor of Japan)
'novel revolutionary trouble' is the wordplay.
'novel' becomes 'kim' (Rudyard Kipling novel).
'revolutionary' shows that the letters should be reversed in order.
'trouble' becomes 'ado' (both can mean bother or commotion).
'kim' reversed gives 'mik'.
'mik'+'ado'='MIKADO'
(Other definitions for mikado that I've seen before include "Japanese emperor - Gilbert and Sullivan character" , "G & S' alternative for "The Town of Titipu"" , "Japanese emperor, operetta and delicious biscuit" , "mperor" , "Japanese emperor's title" .)