clean it with a soft cloth.

yes you clean up after a flood

Clean shaven is much better clean shaven for most people, sometimes a beard works.

Yes, clean slate is an expression. The official definition for the word clean slate is "an opportunity to start over without prejudice."

The principle is 'dilution': with the first addition of clean water to dirty water, the dirty water becomes less dirty because it is diluted by the clean water. With the continued addition of clean water to dirty water, eventually the dirty water will be completely diluted and there will be no dirty water left; it will all be replaced by clean water.