The ten clues marked * have a shared (omitted) definition, and *** refers to those ten answers collectively

Across:
   7. A vital development * (6)
   9. Tourists love Niagara region * (8)
  10. Let sit to brew, it can be deadly (8)
  11. I halt stranger holding "Girl on Fire" (6)
  12. Confirmed deviant I've fired (8)
  13. Lead astray in Treaty of Accession for instance! (4)
  15. Desperate one invaded * (7)
  17. Century with Chinese and Indian leadership amidst strange haze * (7)
  20. Fancy fellow among returning dignitaries (4)
  21. Ask, and voila! Arranged! * (8)
  24. Legacy Prussian territory? * (6)
  25. Getting together and sexually clutching, having 23? (8)
  26,23dn. Australian native's green sister is first on 25, joined by *** in 2003 13 (8 5)
  27. Army helping the needy with some heavy artillery fire (6)
Down:
   1. One chasing a beverage * (5)
   2. Take on new form of Swedish vehicle mostly dividing 26 leaders (6)
   3. Dubai not set up to provide * (7)
   4. Do plan changes! * (6)
   5. Appears in old itch regularly coming up, every seven years perhaps? (8)
   6. Haul it in a wreck * (9)
   8. Badly ire star, being even more creative (7)
  14. Work in prunus cultivation as a way to secure food (6 3)
  16. Official ruling over Norway region (8)
  18. In Belize, a lot suffer fanatics (7)
  19. Dig building in pine for somewhere to stay (7)
  21. Program ... one that works, ideally, and ties ends up? (6)
  22. I broke maple stick (6)
  23. see 26 across