Tropical freshwater fish (5)
I believe the answer is:
tetra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tropical freshwater fish</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" , "Denoting four (prefix)" , "Small brightly-coloured fish of aquariums" , "Wrongful act (Law)" , "Bright fish; four (prefix)" .)