The most common and most likely answer to this crossword clue is the 13 letter word FREDERIKSBERG
I've seen this crossword clue in Financial Times
F
R
E
D
E
R
I
K
S
B
E
R
G
Buddy explains!
The straight part
We know that "European municipality" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case european municipality = frederiksberg.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.