Etcher's need (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">etcher's need</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 "Caustic and biting" , "Sarcastic - tart" , "Bitter substance" , "Substance which turns litmus red" , "something sharp" .)