On an I-pad you have to use the icons since they don't have command and control buttons. The first "x" icon marks the field blank. The second "x" icon marks the whole entry blank. That is sometimes very useful when you only have a couple of fields to enter data in.
Only if you have a Bluetooth keyboard attached to and paired with your iPad will you be able to use the keyboard shortcuts. Otherwise, as Melissa mentioned, you need to use the horizontal toolbar.
Here is a gipuide to the horizontal toolbar. Some of the shortcuts mentioned don’t work anymore. I need to update it for that, but the descriptions are OK.