Complete
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7 9 13 16 5 18 1 10 2 8 15 14 3 12 11 4 17 6
  1. 5a Unpleasant clash (11)
  2. 7a Air pollution common in big cities (4)
  3. 8a Post holder (8)
  4. 9a MC on stage and screen (7)
  5. 11a Bishop's hat (5)
  6. 13a Confused (by life afloat?) (2,3)
  7. 14a Term of endearment (3,4)
  8. 16a Small stirrer — not Aesop! (anag) (8)
  9. 17a Make a reservation (4)
  10. 18a Nicest peers (anag) — tenacious behaviour (11)
  11. 1d Hitch (4)
  12. 2d Outstanding amount to pay (7)
  13. 3d Where to write notes, musically (5)
  14. 4d Water-based paint (8)
  15. 5d What's left to rot in a garden (7,4)
  16. 6d Put one image over another (11)
  17. 10d (Exert) force (8)
  18. 12d A paltry sum of money (informal) (7)
  19. 15d It's uplifting (5)
  20. 17d Like a skeleton (4)