Do the followin' if you want to catch User's hit on Clear button.
Also can be usefull with many trigger actions.
1. Create a variable vClearButtonHit;
2. Set this expression for it:
[this condition will increase variable value till it under 10, after that the value will be droped to zero]
3. Make a trigger on changing this variable (Ctrl + Alt + D -> Triggers).
In attached example trigger is set for selecting 'x' value in f3 field on Clear Button hit.
Sometimes it can be usefull and it's an analogue of "One Selected Value" option.