Defensive building (4)
I believe the answer is:
fort
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">defensive building</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 fort that I've seen before include "Defensive building" , "castle, perhaps" , "Keep" , "Garrison" , "Strategic position" .)