Water tank (7)
I believe the answer is:
cistern
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">water tank</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cistern that I've seen before include "bit of toilet" , "Toilet water reservoir" , "Water storage tank" , "Water container" , "Tank - sac of body fluid" .)