Make less transparent (5)
I believe the answer is:
cloud
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">make less transparent</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 cloud that I've seen before include "Mass of water droplets" , "It hides the sun" , "Obscure" , "Nebula" , "Dull spot" .)