As was noted in an earlier entry, the word all means “the good life.” The phrase “all clear” refers to a state of being that is even better than the good life.
All clear is the state of living the good life on a clear day, which is, for most people, a notch up on the enjoyability scale from living the good life on, say, a cloudy day, a rainy day, a snowy day or, particularly, in the midst of a category five hurricane. Although, it’s hard to imagine that anyone could be considered to be living the good life in any respect in the midst of a category five hurricane, so some would consider the latter example of a non-all-clear state to be nonsensical.
All clear also has a second meaning that applies only in the field of medicine. In medical field, all clear is the clinical, scientific term that is used to indicate that a prescribed acne remedy was effective.