Margin marking (4)
I believe the answer is:
stet
'margin marking' 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 "Leave in, as text" , "'Let it stand, in printing (4)'" , "As written, don't change" , "Eg, molar" , "leave things as they were" .)