Castle's defense (4)
I believe the answer is:
moat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">castle's defense</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 moat that I've seen before include "Water crossed via a drawbridge" , ". . . part of defence" , "Trench round a castle" , "Water round a castle" , "Water-filled ditch around stronghold" .)