Engrave with acid (4)
I believe the answer is:
etch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">engrave with acid</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 etch that I've seen before include "Make carvings" , "Cut; engrave" , "Engrave (metal, glass etc.)" , "Design on metal or glass using acid" , "Wear away by chemical reaction" .)