ACROSS
 1. S_E ←SECRETS← = SESTERCES <ancient Roman coins>
 6. + S Y SOP = SYSOP <network administrator>
 9. <Russian leader> VOZHD = V OZ + HD
10. _N AU (MACH 1) + A = NAUMACHIA <sea battle>
11. <siren> LORELEI = LORE + _L_E_I_
12. <solvent> TOLUENE = TO L_ _U_ ENE
13. RED INGOT _E = REDINGOTE <overcoat>
16. P HIS H_ = PHISH <scam online>
18. W U + XI A = WUXIA <adventure movies set in China>
20. ME ←L PORT← + EX = METROPLEX <urban area>
22. <break> CAESURA = C_A ←A RUSE←
24. (MY RADAR)* = YARDARM <beam on ship>
26. <agreeably> PIACEVOLE = PI ACE + VOLE
27. <it's worth a couple> TWOER = T_ W_ O_ E_ R_
28. I'D D inside KO = KIDDO <sport>
29. PARAMEDIC* = PRE-ADAMIC <arriving before rib donor>

DOWN
 1. <group of couturiers> SAVILE ROW = ←WORE LIV _A _S←
 2. <Cambridge student> SIZAR = SIR outside ←AZ←
 3. _E IDOL ON = EIDOLON <apparition>
 4. <writing> CUNEIFORM = C UNE + I FOR M
 5. <rubbish> SQUIT = S + QUIT
 6. <rowboat> SHALLOP = SHALL OP
 7. SCH LE_ M IE _L = SCHLEMIEL <an unlucky fool>
 8. ←AH P← RE = PHARE <beacon>
14. I XI inside DEL AND = DIXIELAND <New Orleans jazz>
15. <unsound argument> ENTHYMEME = EN THY MEME
17. HEX AMERIC[-a] = HEXAMERIC <comprising six discrete components>
19. _AZ_ _U_ _L_ JOE* = AZULEJO <pottery tile>
21. (.d)OER ST ED(w.) = OERSTED <amount of magnetic intensity>
22. CAPE + K_ = CAPEK <robot inventor>
23. <jack's back> APOOP = ←POO PA←
25. A ←MOB← outside H = ABOHM <resistance unit>