Drainage channel (5)
I believe the answer is:
ditch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">drainage channel</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 ditch that I've seen before include "Throw away (sl.)" , "Gully or furrow" , "side of field?" , "Scrap" , "Land in water" .)