Substance with a low pH (4)
I believe the answer is:
acid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance with a low ph</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for acid that I've seen before include "It is neutralised by alkali" , "Drug, tart" , "Cutting" , "It is neutralized by alkali" , "Corrosive substance" .)