The most common and most likely answer to this crossword clue is the 6 letter word RUNNEL
I've seen this crossword clue in The Guardian
R
U
N
N
E
L
More clues leading to the result RUNNEL
Buddy explains!

The straight part
We know that "stream" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stream = runnel.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RUNNEL mean?
RUNNEL noun- A small stream, a rivulet.
- To create channels for directing the flow of liquid.