Complete
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  1. 1a Add on at the end (6)
  2. 4a Go faster, horsey! (3,2)
  3. 7a Give birth to piglets (6)
  4. 8a Stylish (6)
  5. 9a Disgusting waste matter (4)
  6. 10a Firmly tethered (8)
  7. 12a What holds the main microchips in a computer (11)
  8. 17a Superior skill (8)
  9. 19a Heavy dull sound (4)
  10. 20a Petrol jelly used in flame-throwers (6)
  11. 21a Supernatural (6)
  12. 22a Sexually attractive? (5)
  13. 23a Vanquish (6)
  14. 1d A ragman (anag) (7)
  15. 2d Gemstone — pre-do it (anag) (7)
  16. 3d Guardian seller? (9)
  17. 4d Diagram showing the relationship between variable quantities (5)
  18. 5d Sovereign (7)
  19. 6d Ceremonial procession (6)
  20. 11d Place for keeping bits and bobs (9)
  21. 13d Sea creature's tentacles served as food (7)
  22. 14d 'The', definitely (7)
  23. 15d Pair of similar things (7)
  24. 16d Hard dark-brown wood used furniture (6)
  25. 18d With a smooth, gleaming surface (5)