Ditch around a castle (4)
I believe the answer is:
moat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ditch around a castle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for moat that I've seen before include "defensive construction" , "Ditch used as a defence" , "A deep water-filled ditch around a castle" , "Last ditch defence?" , "Trench around fortifications" .)