ACROSS
 1. <I hold up wood> ANDIRON = AND IRON
 5. <piece> SONATA = (.i)SON AT A(l.)
 9. PA X = PAX <uncle>
10. (SPLIT SEVILLE) - E = SPLITSVILLE <divorce>
11. <plan> AGENDA = AGE NDA
12. FRAT outside GRAN = FRAGRANT <with bouquet>
14. <leave> EXIT = EX + IT
15. MET A STABLE = METASTABLE <in excited state>
18. (YMCA'S REMIT)* = ASYMMETRIC <different, on reflection>
19. ←EG RU← = URGE <press>
21. <heavenly> ETHEREAL = ALE* outside THERE
23. WALES _A_ = WALESA <president of Poland>
25. <opportunity> CONVENIENCE = CON (EN inside VENICE)
26. ←OK← I = KOI <fish>
27. Q SANTA* = QANTAS <airline>
28. <subatomic particle> NEUTRON = "NEW" TRON

DOWN
 1. (ALPHA BETA) + GEN C Y = ALPHABET AGENCY <FBI?>
 2. <finesse> DEXTERITY = DEXTER←C, CITY
 3. <live> RESIDE = RE SIDE
 4. NIL + _E = NILE <river>
 5. AUSTRALIAN* = SATURNALIA <festival in December>
 6. <steer> NAVIGATE = ←VAN← I + GATE
 7. _T_ USA outside L = TULSA <Oklahoma city>
 8. <50s nonconformists> BEAT GENERATION = (ARE ABETTING ONE)*
13. <animal books> BESTIARIES = BEST I + ARIES
16. <warrior> BERSERKER = (B + ER) (S + ER) (K + ER)
17. <most stylish> SMARTEST = MATTRESS*
20. ←TAG← + EAU = GATEAU <cake>
22. ←AD NOH← = HONDA <Japanese car>
24. <aware of> IN ON = ↺NINO↺