"Clear warning" means a direct and easily understandable indication that something risky or dangerous might happen in the future, so it's important to take caution or make changes. Full definition
Yet, four out of five teens who attempt suicide have given clear warning signs. (edutopia.org)
However, sometimes these car accidents can involve errors on the part of the construction companies and failure to give drivers clear warning of dangerous conditions. (fortlauderdalecaraccidentattorneyblog.com)
Give clear warning, make sure everyone knows that such behavior will not be tolerated, and then act quickly. (rismedia.com)