Engrave on glass (4)
I believe the answer is:
etch
'engrave on glass' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for etch that I've seen before include "What most engravers presumably find easy to do" , "Make a deep impression" , "Delineate" , "Corrode" , "Engrave (metal/glass)" .)