Tropical freshwater fish (5)
I believe the answer is:
tetra
'tropical freshwater fish' is the definition.
(I've seen this in another clue)
This is all the clue.
(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)" .)