Widen a margin (6)
I believe the answer is:
indent
'widen a margin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for indent that I've seen before include "Notch on edge" , "Requisition for stores" , "Official order" , "Start line further in from page margins than others" , "Make a purchasing order - place away from the margin" .)