Clue: More work required on song that's included with German opera (6,7)
Answer: UTOPIA LIMITED
Explanation: Definition: opera
UTOPIA (work by Sir Thomas More – a popular trick with setters) + MIT (German "with") in LIED (song – I'm not sure whether "German" in the clue is intended to apply to this as well as "with"). Utopia Limited is one of the lesser-known and less-successful works of Gilbert and Sullivan