Clue: Führer's edict ruined unknown opera (3,10)
Answer: DER FREISCHÜTZ
Explanation: Definition: opera
(FÜHRER'S EDICT)* + Z (an unknown, in algebra) for the opera by Weber, very popular in its time but (I think) not often performed nowadays. It's rather pleasing that, for once, the umlauted U appears in both the anagram fodder and the answer