Not translucent (6)
I believe the answer is:
opaque
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not translucent</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 opaque that I've seen before include "Not allowing light to pass through" , "Dark" , "Cloudy, non-transparent" , "Mysterious" , "Hard to understand, see through" .)