Outline sharply (4)
I believe the answer is:
etch
'outline sharply' 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 "Engrave metal plate using acid" , "Cut lines with acid" , "Carve into surface of something" , "Design on metal or glass using acid" , "Make designs" .)