In the SCA, uttering the word "Clear!" in a loud, clear voice is a way of warning others that you are holding or drawing live steel, like a sword or dagger. Using the word "clear" is a required safety measure and a courtesy to others, but is not a replacement for common sense.
"Clear and collect"
"Clear and collect" is an expression used in the SCA to indicate that the field of combat or the archery range is free from danger and that fighters can now go and collect their arrows etc.. It is normally uttered by marshals when they feel everything is safe, e.g. no arrows are nocked, all fighting has stopped.