Qt 5.10 Beta was released this week and the neon builder cloud elves have been compiling it away ready for testing.
There’s no QtWebEngine or Qt3D so stuff which needs those will be broken.
Other stuff likely broken too, don’t use it on a machine you’re not happy doing a reinstall on.
But the good news is the broken clock plasmoid works now 🙂
deb http://archive.neon.kde.org/testing xenial main