The most common and most likely answer to this crossword clue is the 12 letter word TRICHROMATIC
T
R
I
C
H
R
O
M
A
T
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C
Buddy explains!

The straight part
We know that "Like French flag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like french flag = trichromatic.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 TRICHROMATIC mean?
TRICHROMATIC adjective- Involving three colours.
- Able to perceive three primary colours.