Grumbler, that is so convoluted, I can't even make out what it means)))

"clean" isn't where they pick up litter, but where they DON'T litter (in the first place). . . is that what this means?

Yes
The point is that for a place to be clean people have to stop littering in the first place.

But beyond that, they’re in a way clarifying what CLEAN signifies, and what it doesn’t, yes?

По смыслу CLEAN здесь очначает "чисто от мусора", отсутствие мусора ("сорят", "убирают" (а не "моют")) - всего, что должно быть брошено в урну, а не на пол/улицу.

Да, это уже понятно, но я про структуру/смысл целого предложения: by CLEAN we mean this and not this.

I don't understand what you mean.
The stress is on the action - not littering is more important than cleaning

Of cause, it's some exaggeration, overstatement - to make the point more clear

What I mean is that the meaning of the entire sentence hinges on the definition of what’s considered clean: it’s not THIS, it’s THAT.

I’m just wondering if I was correct in putting “CLEAN” between quotes the way I did.

In don't think that quotation marks are need in Russian, but in English [clean is where the litter isn't dropped, not where it's picked up] would possibly be better than my original translation.
Does it have to be "clean" in this case?

Yes, because you’re referring to the meaning of CLEAN as an adjective, but using it in a place where you’d normally use a noun.

In this case it's rather an explanation of "how (to achieve)" than the "meaning".

In English too

That’s why i added “in the first place”

I like "in the first place", but in Russian we have "в первую очередь" too.
Так что, исправить на
"clean" is where the litter isn't dropped, not where it's picked up
?

I think it would be better if you just add your variant.

👍🏼