I know it is possible to clear the format of a cell with {Backspace}. However, you must first press {Backspace} a first time and then select what attributes you wish to clear. Each subsequent instances, once your choice is made, you must press {Backspace}, then {Enter} (2 Steps), and you must define which attributes you wish to clear on each new document.
In LibreOffice Calc, I looked in the “Customize/Keyboard” Options for “Clear” and “Default” but didn’t find anything that can accomplish this in (a “1-Stop” Hotkey press).
Thanks!