all clear

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A signal, usually by siren, that an air raid is over or a danger has passed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a signal that danger, such as an air raid, is over; usually a siren at a single pitch
• noun permission to proceed after obstacles have been removed

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a signal (usually a siren) that danger is over
• noun permission to proceed because obstacles have been removed