Complete
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7 9 16 17 20 22 1 12 13 2 18 3 10 23 8 11 21 4 14 19 5 15 6
  1. 1a Chirping insect (6)
  2. 4a Italian island where Gracie Fields lived for the last part of her life (5)
  3. 7a Fruit in a tarte Tatin (6)
  4. 8a Event that is not repeated (3-3)
  5. 9a Fly high (4)
  6. 10a Large shop (8)
  7. 12a Goodbye for now (3,3,5)
  8. 17a Underwater swimmer (8)
  9. 19a Lady singer, b. 1986 (4)
  10. 20a Underground passage (6)
  11. 21a Kind of beer (6)
  12. 22a Assorted (5)
  13. 23a Written (6)
  14. 1d Plentiful (7)
  15. 2d Unit of heat energy (7)
  16. 3d Othello's wife (9)
  17. 4d Africa's second longest river (5)
  18. 5d Supply (7)
  19. 6d Notoriety (6)
  20. 11d Tasty (9)
  21. 13d Annual moments when day and night both last about 12 hours everywhere (7)
  22. 14d Screw up (7)
  23. 15d Plant of the daisy family — Edgware (anag) (7)
  24. 16d Large metal gong (3-3)
  25. 18d Very much (informal) (2,3)