Battery liquid (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">battery liquid</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 acid that I've seen before include "Bitter - drug" , "Drug LSD (colloq.)" , "- - -drop, sweet" , "Sour - vitriolic" , "Corrosive fluid" .)