Prior art for an icon for "clear-all-fields?"

That's a hard one. When there's no standard it's always tricky to be the canary in the cage. I'm guessing a word is out of the question.

Whatever icon you go for, be sure to test it. In my experience, many discovery problems come down ambiguous or vague icons

Jared Spool has 3 short rules about icon choices: http://www.uie.com/brainsparks/2009/06/28/old-news-about-ico... worth a read.