Defensive ditches (5)
I believe the answer is:
moats
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">defensive ditches</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for moats that I've seen before include "Water-filled ditches as fortifications" , "Trenches around castles" , "Defensive water-filled ditches" , "Ditches around castles" , "Water defences" .)