Castle surround (4)
I believe the answer is:
moat
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">castle surround</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 "Ditch used as a defence" , "Last ditch defence?" , "defensive construction" , "Water around castle" , ". . . part of defence" .)