Thick mist (3)
I believe the answer is:
fog
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thick mist</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 fog that I've seen before include "Miasma" , "Obscure" , "second growth of grass" , "Heavy mist" , "Impenetrable mist" .)