Complete
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7 14 19 1 9 12 21 2 15 3 11 20 10 4 18 16 5 8 13 6 17
  1. 1a Large quantity (informal) (10)
  2. 7a Unvarying (8)
  3. 8a Boy's name — academic bigwig (4)
  4. 9a Major tributary of the Rhine, flowing through Frankfurt (4)
  5. 10a Dazed (7)
  6. 12a Fixed and definitive (4,3,4)
  7. 14a Tolerate (7)
  8. 16a Run like billy-o (4)
  9. 19a Deliberate trick (4)
  10. 20a Very soon (2,1,5)
  11. 21a It's not a good place to get stuck! (2,1,3,4)
  12. 1d Flower (5)
  13. 2d Employee taking notes? (7)
  14. 3d Distinctive periods (4)
  15. 4d Scope for freedom of action (8)
  16. 5d Accessory (3-2)
  17. 6d Natural ability (sought by scouts?) (6)
  18. 11d Mending — nightcap (anag) (8)
  19. 12d Car modified for extra power and speed (3,3)
  20. 13d Studio — I relate (anag) (7)
  21. 15d Confuse (3,2)
  22. 17d Ransack (5)
  23. 18d You're getting closer! (4)