Water supply (9)
I believe the answer is:
reservoir
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">water supply</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 reservoir that I've seen before include "Lake created to store water" , "Area flooded" , "Large water store" , "Man-made lake or tank" , "Lake for collecting and storing water" .)