Brick barrier (4)
I believe the answer is:
wall
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">brick barrier</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 wall that I've seen before include "See 1" , "Built barrier" , "Side of building" , "Brick/stone barrier" , "A challenge for the mountaineer" .)