The clear action button will be disabled if you don't have any branches selected, or if your map is in Gantt mode.
You can tell if your map is in Gantt mode by looking at the ribbon. If there is a green tab called Project" about the "Gantt" tab then your map is in Gantt mode.
You are prevented from clearing action properties in Gantt mode because it would cause potentially unexpected things to happen to your chart.
If you are finished working with the Gantt chart you can exit Gantt mode by switching to the "Gantt" ribbon, opening the "Return to Map" menu and selecting "Exit Project Mode".
Project in Gantt ModeExit Gantt Mode and Return to Map