River horses (6)
I believe the answer is:
hippos
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">river horses</span>' is the definition.<br/ ><span class="explanation-format__explanation">('hippo' can be a synonym of 'river horse')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hippos that I've seen before include "River horses" , "African natives" .)