How to clear unit selection in Starcraft 2?

No, Just like SC1, there is no way to 'Deselect'. What you must do is select a building or other unit. Or let your selected units all die.

If you want to deselect all your units, just click another unit or let said unit die. You can also select an enemy unit and as soon as you lose vision, you'll have nothing selected.

If you accidentally add a unit to your current selection , you can shift+click said unit and it will be de-selected from your current selection.

You can also shift+ctrl+click a unit to de-select all of this unit type from your current selection.

These tricks are pretty handy when you accidentally select workers at the same time of your army or mix your spell casters with your main army by accident.

Shift-click adds or removes a unit or building to current selection.

Hence, you can shift-click the only selected unit or building to empty your selection.

It is possible to remove units via shift-clicking or shift-ctrl-clicking their portraits in the bottom panel, but just clicking and then shift-clicking any single unit seems easier and more unconditional.