### Clue: Operetta's spoiled by my libretto? (9)
### Answer: POETASTER 
 ### Explanation: Definition: Operetta's spoiled by my libretto
  **Anagram of**(spoiled) OPERETTAS
Defn:  A poetaster,poor poet would surely spoil an operetta if he/she were to write the libretti for it.  My COD.  Clever intertwining of definition and wordplay.