Make an aquatint (4)
I believe the answer is:
etch
'make an aquatint' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for etch that I've seen before include "Carve into metal with acid" , "Engrave (metal, glass etc.)" , "Eat away with acid" , "Score deeply" , "Cut (design) on surface" .)