Boundary earthwork (4)
I believe the answer is:
dyke
'boundary earthwork' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dyke that I've seen before include "Flood defence" , "Barrier against water" , "Barrier to prevent flooding" , "Ditch; embankment" , "Embankment built to prevent flooding as in Holland" .)