The most common and most likely answer to this crossword clue is the 7 plus 5 letter word SLUMBER PARTY
I've seen this crossword clue in The Sunday Telegraph
S
L
U
M
B
E
R
P
A
R
T
Y
Buddy explains!
The straight part
We know that "American social event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case american social event = slumber 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.