Fish in streams (4)
I believe the answer is:
rays
This clue is a double definition.
'fish' is the first definition.
(I've seen this before)
'streams' is the second definition.
(ray can mean a stream or beam)
'in' acts as a link.
(Other definitions for rays that I've seen before include "shafts" , "Beams of sunlight" , "Columns of light or heat" , "Sunbeams" , "Fish with wing-shaped fins and long tails" .)