Clue: Theatre upset with composition involving new organ, sadly undesirable (7,3,5)
Answer: PERSONA NON GRATA
Explanation: Definition: undesirable
A charade of PER, a reversal ('upset') of REP ('theatre') plus SONANONGRATA, an envelope ('involving') of N ('new') plus ONGRA, an anagram ('sadly') of 'organ' in SONATA ('composition').