Prefix meaning "four" (5)
I believe the answer is:
tetra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prefix meaning four</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 tetra that I've seen before include "... Four < , in composition" , "in groups of four" , "Small brightly-coloured fish of aquariums" , "Swimmer" , "Small bright fish" .)