Dirty air (4)
I believe the answer is:
smog
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dirty air</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for smog that I've seen before include "Thick polluted mist" , "Pollutant haze" , "Fog and smoke mix" , "Fog polluted by smoke" , "Industrially polluted air" .)