Small river (7)
'small river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stream that I've seen before include "schoolchildren grouped together" , "group of schoolchildren" , "play in real time" , "Brook or rivulet" , "'Current, flow (6)'" .)
(Other definitions for rivulet that I've seen before include "burn" , "Vile rut for small stream" , "Brook" , "Big stream" , "fast stream" .)