If the problem was the classified nature of the answer, the proper answer would have been something like "The law prvents me from addressing classified topics in public", not "No... not wittingly". Given that it was congress doing the asking, they would have had any number of ways they could have addressed that concern including giving him immunity or cleared the chamber so he was only addressing congress. Also, as Clapper was given the questions beforehand, he had ample opportunity to bring this up beforehand.