Castle’s ditch (4)
I believe the answer is:
moat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">castle's ditch</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for moat that I've seen before include "Water round a castle" , "Defensive water" , "Defensive water-filled ditch" , "Protective ditch" , "Trench around fortifications" .)