Complete
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8 10 13 16 6 5 19 1 17 2 9 14 3 11 15 4 12 18 7
  1. 5a Heights above sea level (9)
  2. 8a Essence (sometimes hard) (4)
  3. 9a Person from whom one is descended — no traces (anag) (8)
  4. 10a Jagged (6)
  5. 11a Sheen (6)
  6. 13a Teeth straighteners (6)
  7. 15a Binding agent (6)
  8. 16a Ambiguity providing a way of avoiding a legal obligation (8)
  9. 18a Bird of peace (4)
  10. 19a Place to live (9)
  11. 1d Likely to react badly? (8)
  12. 2d Numbering system using only 1 and 0 (6)
  13. 3d Part of a car wheel (6)
  14. 4d FBI agents (4)
  15. 6d Where company directors get round the table (9)
  16. 7d The Girl from Ipanema's dance? (5,4)
  17. 12d Weak and ineffectual person (4,4)
  18. 14d Unemotional (6)
  19. 15d Ingenious (6)
  20. 17d That was a narrow thing! (4)