Great flood (6)
I believe the answer is:
deluge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">great flood</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for deluge that I've seen before include "Overwhelm with water" , "A heavy fall of rain (6)" , "Inundation" , "Severe flood" , "Torrential rain" .)