Across: 1. semipro 5. chippie 9. own up 10. funny bone 11. guerrillas 12. peck 14. raison d’etre 18. mississippi 21. rout 22. clean sweep 25. string-bag 26a/2d. brass monkey 27. absence 28. needles.
Down: 1. spongy 2. see 26a 3. paper trail 4. offal 5. cantaloup 6. idyl 7. property 8. see 24d 13. admissible 15. insoluble 16. ambrosia 17. esquires 19. detail 20. spasms 23. argon 24d/8d. on an even keel