The most common and most likely answer to this crossword clue is the 9 letter word ORIFLAMME
I've seen this crossword clue in The Times
O
R
I
F
L
A
M
M
E
More clues leading to the result ORIFLAMME
- Touring Rhode Island, old lover carries minute scarlet flag
- Standard old gun charged with bullets, sawn-off
- Standard type of blood a short distance inside arm
- Standard ordnance content’s replaced by sawn-off rifle and ammo
- Standard Italian coastal region a non-starter up in Rome, perhaps?
- Standard of France in ruined memorial
- Standard memorial, fluttering, borne by French leader
- A banner at new memorial displaying force
Buddy explains!

The straight part
We know that "scarlet flag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case scarlet flag = oriflamme.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 ORIFLAMME mean?
ORIFLAMME noun- (history) The red silk banner of St Denis, which the abbot of St Denis gave to French kings as they rode to war.
- Any banner, idea or principle which serves as a rallying point for those involved in a struggle.
- Something resembling the banner of St Denis; a bright, shining object.