Fort (6)
I believe the answer is:
castle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fort</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for castle that I've seen before include "Make chess move involving king and rook" , "Lincoln, Ludlow or Leeds" , "ancient building?" , "Dover, perhaps" , "man of 1d [ADVENTURE]" .)