Cardiac organ (5)
I believe the answer is:
heart
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cardiac organ</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 heart that I've seen before include "Core; organ" , "Not a club" , "Cardiac courage" , "Spirit - core" , "Muscular organ for blood circulation" .)