Polluted air (4)
I believe the answer is:
smog
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">polluted air</span>' is the definition.<br/ ><span class="explanation-format__explanation">(smog is fog caused by pollution)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for smog that I've seen before include "Pollutant fog" , "Industrially polluted air" , "Pollution haze/fog" , "Dirty mixture of smoke and fog" , "Pollution fog" .)