I have a very much customized Plasma 5.8 Desktop.
Now i wonder if there is a clean way to save/apply all these desktop settings to another machine, same type, same installation.
That is, do I find some export/import of settings?

Thanks in advance,

Uwe

There is no export/import function (that I know of).

You could copy:
~/.config
~/.local
~/.kde4 (yes, a few things still may use it)
from your source machine to the destination. Make the copy whilst logged in to a VT, not from within KDE itself.

That should carry most, if not all of your customisations, provided you've not added anything over and above a standard install.