The most common and most likely answer to this crossword clue is the 11 letter word BURGOMASTER
I've seen this crossword clue in The Times
B
U
R
G
O
M
A
S
T
E
R
More clues leading to the result BURGOMASTER
Buddy explains!

The straight part
We know that "European mayor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case european mayor = burgomaster.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 BURGOMASTER mean?
BURGOMASTER noun- The mayor, or head magistrate, of a town in the Netherlands, Belgium, Germany and certain other countries.