Blue streamer? (6)
I believe the answer is:
danube
'blue streamer?' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
This is all the clue.
Can you help me to learn more?
(Other definitions for danube that I've seen before include "Ban due (anag) - river" , "5 down's river" , "Vienna's river" , "South European river" , "River flowing from Black Forest to Black Sea" .)