The most common and most likely answer to this crossword clue is the 6 plus 2 plus 9 letter word ANGELS ON HORSEBACK
A
N
G
E
L
S
O
N
H
O
R
S
E
B
A
C
K
Buddy explains!
The straight part
We know that "An hors d'oeuvre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an hors d'oeuvre = angels on horseback.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.