Dele undoer (4)
I believe the answer is:
stet
'dele undoer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stet that I've seen before include "Proofreader's mark that the original text should be kept" , "Let it remain (printing)" , "Word to cancel change" , "Let it stand (editing mark)" , "Print as is, don't alter spelling" .)