WC water tank (7)
I believe the answer is:
cistern
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wc water tank</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 cistern that I've seen before include "Water tank for toilet" , "Tank - sac of body fluid" , "Tank, reservoir" , "Water storage tank" , "Vat" .)