Reservoir (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">reservoir</span>' is the definition.<br/ ><span class="explanation-format__explanation">(source of water)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tank that I've seen before include "Large storage chamber" , "defeat" , "Offensive vehicle" , "Armoured vehicle - container for liquid or gas" , "Storage chamber; drink heavily (colloq.)" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">reservoir</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dam that I've seen before include "eg Aswan" , "Weir" , "Horse's mother" , "River blockage" , "River restraint" .)