Puns.

Across clue definitions are all puns. E.g.
Clue: Troubled Roman bearing (5)
Solution: MANOR (pun on MANNER).

Across:
7. Doubtful dogma, bell took a chance (9)
8. Origin of lost cause (5)
10. Sexual misfits (DC) round water, with energy (8)
11. Sin badly with racy set in groups (6)
12. Exist where sugar comes from (4)
13. See round all-round sailor, can be stamped (8)
15. A playing-field is a silver circle (7)
17. Away with last missing ruler. An arousal (7)
20. Disparages legs, this mess (8)
22. Ill-gotten gains left with American truck (4)
25. Donkey top-class. Left an exchange (6)
26. In backward ego hides morning on fire (8)
27. Loud exclamation backing south-east streams (5)
28. At an area of land or a farm vehicle (9)

Down:
1. Gasped and paged wildly (5)
2. Do a favour for old boy left with a vestige, but no coat (6)
3. Board with weight? Minute life-form (8)
4. Death in December, free from pain (7)
5. Farmer's store is a dreadful, shaky act (8)
6. Fuel expert puts the French in northern river (9)
9. Mona's other half ails badly (4)
14. Yes, legals suffer, never growing old (9)
16. Badly-weathered ring, eleven grew old (8)
18. Conflagration destroys if wilder (8)
19. Thirsts hopelessly for casual wear (7)
21. Possess heroin long ago (4)
23. That church has picturesque roof (6)
24. Store held inside potting shed (5)