Mortar (6)
I believe the answer is:
cement
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mortar</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 cement that I've seen before include "Binding substance as for bricks" , "Brick fixer" , "It hardens when mixed with water" , "Union" , "Powder substance to make concrete" .)