Engrave using acid (4)
I believe the answer is:
etch
'engrave using acid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for etch that I've seen before include "Carve into metal with acid" , "Make incisions" , "Make carvings" , "Carve or engrave into surface" , "Cut deeply (with acid?)" .)