The 
Clear Page button clears all ink and text in the main window. If you have multiple tabs open in the window, it only clears the current tab.
You can also use the clear gesture to clear the window:
To enter this gesture, just write a single stroke straight up, away from other text or ink on the display.
Clear Page also available through the Edit menu.
If you want to erase only part of the display, such as a single region or part of a region, use an editing operation or one of the Clear options on the Edit menu. To erase all or part of a region, the easiest approach is usually to select what you want to erase and then cross it out.