Culvert (5)
I believe the answer is:
drain
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">culvert</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 drain that I've seen before include "Draw off U sewer" , "Make weak" , "Nadir (anag.)" , "Waste channel/pipe" , "See 11 dn" .)