Engraved with acid (6)
I believe the answer is:
etched
'engraved with acid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for etched that I've seen before include "Cut or impressed into a surface" , "Carved on metal with acid" , "Made lines on metal with acid" , "made some kind of score" , "Engraved, eg, by acid" .)