ACROSS
6. A + SEA(-l) = ASEA <on a boat>
8. ←(-sle)EPYH(-ead)← = HYPE <promotion>
10. LIE outside V = LIVE <in real time>
11. (-authoritaria)N EXT(-remists) = NEXT <following>
12. "RUE" = ROUX <sauce ingredient>
13. Y(-o)U L(-eav)E = YULE <holiday period>
16. ←(-innov)ATIVE← = EVITA <musical>
18. <broaden> WIDEN = ←IW← + DEN
20. <baking> DRYING = R inside DYING
21. GMOS outside (I + Z) = GIZMOS <devices>
23. DENIED* = INDEED <of course>
25. ←(LAIN + EX)← = XENIAL <hospitable>
26. K(-y)R(-g)Y(-z) P(-e)T(-r)O(-l) + N = KRYPTON <gas>

DOWN
1. ME ←CAVED← = MEDEVAC <rescue>
2. F UNCTION = FUNCTION <work>
3. <historical period> JURASSIC = CUIRASS* after J
4. <fourth> QUADRANT = (A DR) inside QUANT
5. <wrong> UNLAWFUL = U + ⭯LAWN⭯ + F U L
7. <batmen> ORDERLIES = SOLDIER* outside ER
9. <mournful> PLAINTIVE = P + (AIN'T inside LIVE)
14. <banality> TRITENESS = INTERESTS*
15. GI inside (Z OO + LOST) = ZOOLOGIST <scientist>
17. <astronaut> SPACEWOMAN = SPAN outside (ACE WO M)
19. <one imaging> VISUALIZER = VIRALSIZE* outside U
22. ¿?when a monarch [butterfly]'s agitation [flapping] leads to apparently-unrelated downfall [bad weather]¿? = BUTTERFLY EFFECT
24. C + (V COOL ASHKENAZI)* = CZECHOSLOVAKIAN <from former European nation>