Red card (5)
I believe the answer is:
heart
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">red card</span>' is the definition.<br/ ><span class="explanation-format__explanation">(hearts are red in a deck of cards)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for heart that I've seen before include "Pumping organ" , "Muscular organ - compassion" , "the name of a prison in Midlothian!" , "Core; organ" , "Centre; valour" .)