<< I don't know if you can...
It's sort of like the search function, you can't really clear it...
I guess if something *bad* was to have gotten in there, you can always run other run commands until that specific command was wiped off the drop down... >>
Oh, you wanted to clear an entry. Well like Paulson said I don't think there is a way. Follow his suggestion if you only want to clear a "bad" entry. That's the only way I know of.