Warning light (6)
I believe the answer is:
beacon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">warning light</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for beacon that I've seen before include "Warning light on hill or cliff" , "Warning light on hilltop" , "Guiding device - signal fire" , "Signal light on hill" , "warning to others" .)