Acid neutraliser (6)
I believe the answer is:
alkali
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">acid neutraliser</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 alkali that I've seen before include "bases" , "Compound turning litmus blue" , "Chemical base" , "Eg, lime or caustic soda" , "Non-acid" .)