The most common and most likely answer to this crossword clue is the 8 letter word STREAMER
I've seen this crossword clue in The Times
S
T
R
E
A
M
E
R
More clues leading to the result STREAMER
- Headline: 'Ship has crossed river'
- Arrest me for waving flag
- Pennant, perhaps right to be carried by older ship
- King trapped in vessel leads to banner headline
- Flag officer's base aboard ship
- Ship carrying right pennant
- Flag displayed by queen on board ship
- Flag raised initially in ship
- Flagship boarded by king
- Flag flown by ship crossing river
Buddy explains!

The straight part
We know that "Flag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flag = streamer.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 STREAMER mean?
STREAMER noun- A long, narrow flag, or piece of material used or seen as a decoration.
- Strips of paper or other material used as confetti.
- A newspaper headline that runs along the top of a page.
- (heading) In computing.
- In fly fishing, a variety of wet fly designed to mimic a minnow.
- One who searches for stream tin.
- A stream or column of light shooting upward from the horizon, constituting one of the forms of the aurora borealis.