Can we change the default font? (for cutting or clearing, not new workbooks)

To change the default in an already-existing file, go to the 'Page Layout' tab on the ribbon, click 'Fonts' and choose the theme which most closely matches what you want - probably the top 'Office' theme will work for you (Cambria for headings, Calibri for text).

That only changes the font type, not the size. If you want to change the default font size as well, then go to the Home tab, look in the 'Styles' section where you'll find the 'Normal' style. Right-click and select 'Modify' to allow you to change the font, font size, alignment, etc, etc.

The two methods are independent but related - depending on exactly what you want for each file, you might use just the first, just the second or both.

(Ironically, I have the opposite issue - my organisation mandates the use of Arial, but the network admins have left the default as Cambria/Calibri, so I constantly have to change the defaults to what you want to get away from!)