Beaver barrier (3)
I believe the answer is:
dam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">beaver barrier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dam that I've seen before include "A mad kind of mum" , "Retaining barrier" , "Barrier to hold back water" , "Flood barrier" , "River blockage" .)