Castle ditch (4)
I believe the answer is:
moat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">castle 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 "Trench round a castle" , "Ditch round castle" , "A deep water-filled ditch around a castle" , "Defensive water" , "Water-filled defensive trench" .)