The most common and most likely answer to this crossword clue is the 12 letter word FLEUR-DE-LYS
I've seen this crossword clue in The Guardian
F
L
E
U
R
-
D
E
-
L
Y
S
Buddy explains!
The straight part
We know that "Iris" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case iris = fleur-de-lys.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 FLEUR-DE-LYS mean?
FLEUR-DE-LYS noun- A design representing a flower whose three petals are joined together at the bottom, often used in heraldry, where it is particularly associated with the French monarchy.