Boundary earthwork (4)
I believe the answer is:
dyke
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">boundary earthwork</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)