Rook (6)
I believe the answer is:
castle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rook</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that rook can be written as castle)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for castle that I've seen before include "Chess piece - stronghold" , "Fortress; rook" , "ancient building?" , "Fortified place on the chess board" , "Let sac be a chess piece" .)