Complete
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6 8 10 13 16 19 21 7 1 18 17 2 9 15 14 3 12 20 11 4 5
  1. 1a Winning all four major tennis championships in the same calendar year (5,4)
  2. 8a Former PM (5)
  3. 9a Irish river — girl's name (7)
  4. 10a Similarity — portrait (8)
  5. 11a Speech defect (4)
  6. 13a Mouth organ? (6)
  7. 14a Vocalist (6)
  8. 16a Oxfordshire (abbr) (4)
  9. 17a Italian bread (8)
  10. 19a Slanting letters (7)
  11. 20a Spurious (5)
  12. 21a Wading bird — no nutters (anag) (9)
  13. 1d Popular pot plant (8)
  14. 2d Aid (6)
  15. 3d Twofold (4)
  16. 4d Platform for disembarking (7,5)
  17. 5d District of Paris on the left bank (12)
  18. 6d Anti-slavery campaigner (12)
  19. 7d Highly elated (7,2,3)
  20. 12d African country (8)
  21. 15d Debacle (6)
  22. 18d Bad shot (that's as good as a mile?) (4)