is there a workaround to have an inner class with its body defined in some other file in order to not bloat the parent class itself?

You could use a

context(Foo) class Bar

+ a

fun Foo.Bar(unit: Unit = Unit) = Bar()

I cant enable them on MP, can I? https://slack-chats.kotlinlang.org/t/2743606/i-m-trying-to-enable-context-receivers-in-a-multiplatform-pr

What is the reason for inner class use? Maybe there is a pattern that could do something similar with less bloat? if you want the parent class, perhaps you can use inheritance and extend the class?

I'm parsing some format and the inner classes represent groups of logical properties (tens), a couple of these are so much more important than the others that have to be available directly on the parent class. However, they do need some preprocessing

Could you define them in another file and just pass them in? If you have koin you can even inject properties directly

yes, that's what I did