Polluted precipitation (4,4)
I believe the answer is:
acid rain
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">polluted precipitation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for acid rain that I've seen before include "What's ecologically damaging" , "Polluted water" , "Harmful drops" , "Destructive precipitation" , "Devastating drop" .)