I guess it's because "clear" has a distinct sense here. If the subject is a person, then "clear" means "understanding, certain" as in "I'm not very clear on the instructions". Now that I think about it, "on" seems more common and sounds a little better, but I wouldn't rule out "about." But in this sense I would always include one of the prepositions.
Thanks. But I have seen a similar example of "sure" in an English newspaper. He said he was not sure "what they are looking for." Here, the sentence is not impersonal, and "sure" not followed by a preposition. I got confused again.