The most common and most likely answer to this crossword clue is the 9 plus 6 plus 5 letter word EDINBURGH STREET PARTY
I've seen this crossword clue in The Daily Telegraph
E
D
I
N
B
U
R
G
H
S
T
R
E
E
T
P
A
R
T
Y
Buddy explains!
The straight part
We know that "celebration somewhere in Scotland" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case celebration somewhere in scotland = edinburgh street party.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.