The most common and most likely answer to this crossword clue is the 10 letter word TRESPASSER
I've seen this crossword clue in The Guardian
T
R
E
S
P
A
S
S
E
R
More clues leading to the result TRESPASSER
- France’s very outdated on rule for intruder
- She crosses a line in Parisienne’s very old hat, right?
- France’s extremely dated monarch, unwelcome visitor
- 26 French spears dealt with intruder
- Very unfashionable French redhead is an intruder
- Intruder from Quebec’s very out of date, right?
- Right under very old-fashioned Parisian sinner
- One poaches, say, very French old-fashioned starter in restaurant
- One has no right to be on land
- Criminal in France, very old-fashioned royal
Buddy explains!

The straight part
We know that "unwelcome visitor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unwelcome visitor = trespasser.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 TRESPASSER mean?
TRESPASSER noun- One who trespasses; an interloper.