Very much on the easy side, I thought. Good puzzle from Phi as ever. Solving time, 13 mins.
< = reversed * = anagram
ACROSS
5 BESIDE ONESELF Double definition, the second slightly cryptic, hence the “?”
8 ARCHIMEDES This refers, I think, to the story that he raced through the streets naked, saying “Eureka”, I’ve found it – the scientific principle named after him. An ancient Greek.
9 OGRE Hidden <
10 BO(H)R(e)
11 NONPAREILS (person in al)*
12 ROLL-ON ROLL-OFF This helped to make this an easy puzzle for me – answer jumped out from definition and enumeration.
15 TANN (HAUS) ER Opera by Wagner. Tanner = an old sixpenny bit (pre-1971)
17 GI T(h)E
18 S (K) IM Sim = computer game using simulation
18 TRAVELOGUE U (capital of Uruguay) in (great love)* & lit.
20 TH(R)E S (HER S) HARK
DOWN
1 E (SCH) ER
2 I (DO MEN) E O By Mozart. Heard of it, but not familiar with it – the surface reading may well relate to the plot.
3 RED SEA(t)
4 ZERO-ZERO When visibility is so poor that a pilot can see nothing.
6 ORDINARY’S HARES Charge = ordinary (cannot not quite see this, looked in Collins, which is all I have to hand just now)
7 FORK-LIFT TRUCK The cryptic part is, I think, describing how such a truck works, rather than wordplay.
13 LAND MARK Old currency as German marks now replaced by euro.
14 L (ARGent) ESSE(n)
16 A U (‘TIS) M
17 G (LOB) AL
charge=ORDINARY baffles me too.
I think the FORK-LIFT TRUCK is divide=FORK and business=TRUCK” (as in “I’ll have no truck with him”) afer an upturn=”LIFT”
Ordinary: it’s a heraldry thing. The GITHE/ GITE number was lost on me.
Vacated means empty, so you empty the to get te.