The most common and most likely answer to this crossword clue is the 8 letter word BANDEROL
I've seen this crossword clue in The Guardian
B
A
N
D
E
R
O
L
Buddy explains!
The straight part
We know that "Long streamer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case long streamer = banderol.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 BANDEROL mean?
BANDEROL noun- A little banner, flag, or streamer.
- A flat band with an inscription, common in Renaissance buildings.