Low dam across a stream (4)
I believe the answer is:
weir
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">low dam across a stream</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 weir that I've seen before include "Small dam to regulate water flow" , "Low dam built across a stream" , "Madagascan primate" , "fish may be trapped in this enclosure" , "Something that may be seen across river" .)