Useless musical theatre about one revolutionary (11)
I believe the answer is:
inoperative
'useless' is the definition.
(thesaurus)
'musical theatre about one revolutionary' is the wordplay.
'musical' becomes 'evita' (I've seen this before).
'theatre' becomes 'rep' (I've seen this in other clues).
'about one' becomes 'oni' (I can't explain this - if you can you should give a lot more credence to this answer).
'revolutionary' says the letters should be written in reverse.
'evita'+'rep'+'oni'='evitareponi'
'evitareponi' written backwards gives 'INOPERATIVE'.
(Other definitions for inoperative that I've seen before include "It's useless" , "Anna -, Russian poet (Requiem)" , "with no effect" , "Not in working order" .)