Engrave with acid (4)
I believe the answer is:
etch
'engrave with acid' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)