Answer : Clue

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Across
  1. 3. : Prime suspect, perhaps (7)
  2. 4. : Crime branch, abbr. (3)
  3. 5. : Popular mystery genre: ___ noir (4)
  4. 7. : Dark brew (3)
  5. 9. : Short sleep (3)
  6. 10. : Du Maurier classic (7)
  7. 13. : Detective, informally (6)
  8. 14. : Author Chandler’s first name (7)
  9. 15. : Famous Belgian detective (7)
  10. 19. : Folder of evidence (8)
  11. 21. : Poison or pistol, e.g. (6)
  12. 24. : Body found at one (4)
  13. 28. : Harris thriller featuring Hannibal Lecter (15)
  14. 30. : Holmes’s foe, Professor ___ (9)
  15. 31. : Reason for a crime (6)
  16. 32. : Flynn thriller featuring Amy Dunne (8)
Down
  1. 1. : Christie novel aboard a famous train (15)
  2. 2. : Sudden revelation (4)
  3. 4. : Crime investigation locale (12)
  4. 6. : Miss Marple’s creator (7)
  5. 7. : Computer language, briefly (3)
  6. 8. : Morse’s first name (6)
  7. 11. : Card game of chance (8)
  8. 12. : Criminal group (3)
  9. 13. : Espionage agent (3)
  10. 16. : Wander (4)
  11. 17. : Court excuse (5)
  12. 18. : “The Girl with the ___ Tattoo” (6)
  13. 20. : Female sheep (3)
  14. 22. : “___ of Two Cities” (5)
  15. 23. : Holmes’s trusted companion (5)
  16. 25. : “___ the Wolf” (3)
  17. 26. : Confidential source (4)
  18. 27. : Place to plead guilty (4)
  19. 28. : Sleek ocean predator (5)
  20. 29. : Holmes’s address: 221B ___ Street (6)