The most common and most likely answer to this crossword clue is the 3 plus 2 plus 8 letter word ARC DE TRIOMPHE
I've seen this crossword clue in The Times
A
R
C
D
E
T
R
I
O
M
P
H
E
Buddy explains!
The straight part
We know that "Paris monument" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case paris monument = arc de triomphe.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.