ACROSS
 1. (MOST LIED)* inside FORSAKE = FOR OLD TIMES' SAKE <due to nostalgia>
 9. [h]ARMED = ARMED <with a gun?>
10. <plausible> REALISTIC = (ARTICLE IS)*
11. <got in the way of> HINDERED = HIND + DEER*
12. IS TRIAL - L = ISTRIA <Adriatic peninsula>
14. <act as agitator in> ROIL = ROI + _L_
15. I COPER inside COMA = COMIC OPERA <Iolanthe, perhaps>
18. <contemptuous> DISDAINFUL = DI SINFUL outside DA
19. <arid region> GOBI = ←I BOG←
21. RAIDS outside P = RAPIDS <turbulent part of river>
23. <transporter> FERRYMAN = _F _E _R + RYMAN
25. ←ALOE LIME← outside Z = ÉMILE ZOLA <novelist>
26. PARK A = PARKA <weatherproof jacket>
27. [-p]ROB[-e] <=< DISCOVERED CHECK <threat to the king>

DOWN
 1. FEED outside HATER* + FIEND outside R = FEATHERED FRIEND <bird>
 2. ROM[-p] + ANTICS = ROMANTICS <Byron, Shelley, etc>
 3. <women> LADIES = LAD[-d]IES
 4. <undermining> TORPEDOING = (GOOD PINTER)*
 5. <lament> MOAN = MOA + _N
 6. (I'D SPLOSH)* = SLIPSHOD <clumsy>
 7. <flower> ASTER = [-e]ASTER
 8. (A NORMAL CAKE MIX)* outside T = EXCLAMATION MARK <!>
13. ←RE + PUT IV← ATE = VITUPERATE <roast>
16. ECO (MERC inside ME) = E-COMMERCE <trading online>
17. SAND IE GO = SAN DIEGO <California city>
20. <one circling the globe> TROPIC = ←C_ I_ PORT←
22. <proper sort> PRISS = PR IS _S
24. <way of watering> HOSE = "HOSE"