Twelve of the solutions are of a kind, and their clues contain no definition.

Across

1. Get back Gala Cup I spoiled (11)

8. Addiction is hard, to a small degree (5)

9. Plant gold (6)

10. Apply oil to nation, maybe (6)

11. Credit in assistance is sharp (5)

12. I am quiet, catching a number being mischievous (6)

14. Pretends to allow child (4, 2)

15. Twofold removal of first-half bill of sale gives you blisters (5)

17. Beginning to hand Scottish island over (5)

19. Mythical icon lacking story in coarse, printed cloth (6)

22. Sort of music to leave pitch — not second time (6)

24. Man's station wagon (5)

25. Power unit coming back in — old argument's beginning (6)

26. Setter's number returned after an assumed name (6)

27. Spin fabric with sides switched in one place? (5)

28. Left Dick's other half circling reservoir (4, 7)

Down

1. Managed to get het up in the first place (6)

2. Element in “new age” surroundings (8)

3. Somehow aids Celt strongholds (8)

4. Alexander Mocks, say (7)

5. Part of Romeo captured by special drink (7)

6. I'm half of one more than ten, turning up resin (5)

7. A friend's nearly 8 (3, 5)

13. Bringing up a letter after first pet animal (4)

15. “Dizzy,” the Spanish badger (8)

16. Brief departure from the normal barrel — I finally stop (4)

17. Trouble for Washington in being at a higher temperature (3, 5)

18. Ciphers combining Bison and Doe (8)

19. Poise without a penny in bird noise (7)

20. Excluded stupid person carrying a newt (4, 3)

21. Secondary action left in next to salary (6)

23. Awful jealousy after losing bird (5)

Solution