Barker Maths Week Cryptic

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Across
  1. 1. Phase blended into a form (5)
  2. 3. Strange cat burst to make less (8)
  3. 5. Whole number within enceinte German (7)
  4. 6. Mean about 501 is the middle (6)
  5. 9. Dial about argon minus radical joining opposite vertices (8)
  6. 11. An american following a reversing taxi? Can be counted upon. (6)
  7. 13. It is very small, I have hidden it in semifinal (13)
  8. 16. Meatiest mix is my rough guess (8)
  9. 17. Fish in tangled line creates measurement (5)
  10. 18. Side of right triangle crashed phoney utes (10)
  11. 19. First Minister is neither composite nor one (5)
Down
  1. 2. Damaged unpriced pearl stands upright (13)
  2. 4. Sequence in pitter-patter not downpour (7)
  3. 7. Mixing gun tonic finding the total number (8)
  4. 8. Noted actor divides a number (6)
  5. 10. Boundary of shifting empire followed by waterless terrain (9)
  6. 11. Basic mathematics sounds like a rhythmic tick (10)
  7. 12. Novel iconic, fab eponymous series (9)
  8. 14. Foremost, each quantities usually are level (5)
  9. 15. Confusing grey tome reveals a branch of mathematics (8)