River barrier (3)
I believe the answer is:
dam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river barrier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dam that I've seen before include "A mad sort of mum" , "eg Aswan" , "A mad animal mother" , "Mother - barrier" , "Block (river)" .)