One sure sign that National Security Adviser Anthony Lake's job is in jeopardy: Aides are calling reporters to insist that rumors about his being canned by President Clinton are groundless. Staffers say Clinton reassured the former professor in late July that his job was secure. They say the White House plans to raise Lake's profile with appearances on TV news programs and foreign policy speeches in the fall. Still, Lake should not feel too safe. Presidents traditionally back their advisers up until they decide to dump them. Some top Administration officials are betting Lake will be gone by year's end. One official says that at a recent White House meeting, "The President looked annoyed every time Tony spoke."