1st letter of each answer = anagram

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Across
  1. 3. nothing (colloquialism)
  2. 5. chocolate/body of stars
  3. 8. dog or rapper
  4. 9. not odd
  5. 10. pro audio / rat
  6. 15. sound quite atrocious
  7. 18. corn = 1 you = 2
  8. 19. carriages
  9. 20. clown conservative
  10. 21. monkey
Down
  1. 1. muppet
  2. 2. 12/12/19
  3. 4. overground/underground
  4. 6. where the jam goes / rail network
  5. 7. off
  6. 11. what the knights say
  7. 12. hello hello hello hello
  8. 13. dog
  9. 14. dictator / dog
  10. 16. likeness
  11. 17. not guilty