_{This puzzle is a part of the Monthly Topic Challenge #4: Cross-*non*-words}

Are you getting bored with common crosswords? If so, then enable me to deliver this one to your consideration!

Solve the clues and fill the sunshine grey-coloured squares – a part of your process is to work out precisely the way you’re supposed to do this while satisfying this month’s Topic Challenge standards (see the linked meta-post above for extra data)…

Across:
1. )3( tnegA
3. “I’ll return shortly” (3, inits)
4. Acoustic unit of measurement (3)
6. 104 (3)
7. )3( tnuP fo dnaL tneicna eht fo neeuQ
9. 1956 UN conference regarding transportation of cargo by highway (3, inits)
11. )stini ,3( dleifraG drahciR yb detaerc emag drac elbitcelloC
13. )3( !fooW
17. Metal meals container, jail, or bathroom? (3)
18. )stini ,3( “mocimaF” eht sa nwonk osla elosnoc emag oediv emoh tib-8
19. )3( etomiT ni regetni dnoces ro ,nuonorpoeN

Down:
1. Provide with weapons (3)
2. )3( tsepaduB ni esuoH arepO etatS nairagnuH eht fo tcetihcrA
3. “It is” in Latin (3)
5. Former UK commerce union for tutorial employees, merged with NATFHE in 2005 (3, inits)
7. Talk constantly and eagerly (3)
8. Political social gathering in Myanmar, in authorities 2015-2021 (3, inits)
9. )3( reflog lanoisseforp 1 rebmuN dlrow remrof ,___ einrE
10. Common childhood vaccine (3, inits)
12. )3( noiger eniltsaoc nainrofilaC cinecs ,___ giB
14. )stini ,3( ledom ruoloc evitiddA
15. Isotope notation commonplace (3, inits)
16. Düsseldorf-based leisure airline acquired by Air Berlin in 2007 (3, inits)