Dam (4)
I believe the answer is:
weir
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dam</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a weir is a type of dam)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for weir that I've seen before include "Small dam to regulate water flow" , "hazard during 14" , "type of block" , "Feature of river" , "Barrier" .)