Prefix meaning "four" (5)
I believe the answer is:
tetra
'prefix meaning four' 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" , "in groups of four" , "Small brightly-coloured fish of aquariums" , "Swimmer" , "Small bright fish" .)