Very small freshwater fish (6)
I believe the answer is:
minnow
'very small freshwater fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for minnow that I've seen before include "he's unimportant" , "Small fry" , "unimportant person" , "Not much of a catch?" , "Tiny freshwater fish" .)