Top card (3)
I believe the answer is:
ace
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">top card</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ace that I've seen before include "Tennis serve that the receiver is unable to reach" , "Test pilot'" , "Possibly a diamond" , "one among a set of diamonds" , "Virtuoso" .)