Thick paper (4)
I believe the answer is:
card
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thick paper</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 card that I've seen before include "Amusing person" , "Comic" , "Posted greeting" , "Heart or club, say?" , "eccentric character" .)