Cut into metal (4)
I believe the answer is:
etch
'cut into metal' 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 "Cut; engrave" , "Cut design onto metal using acid" , "Inscribe" , "Eat away with acid" , "What most engravers presumably find easy to do" .)