I would like to make a key-binding to select everything below one level above my the current postion of the cursor. This should be the same what happens if I multi-click on an expression -- each click goes one level up. But it would be very comfortable do it on the keyboard.
I have checked the Keyboard Shortcut Listing and Shortcuts package by Rolf Mertig, and it seems to me, that such a key-binding is not readily available. There is an example on how to make a key-binding, which would change color of the select text to get started.
There are some more general instructions in these answers: