The most common and most likely answer to this crossword clue is the 6 plus 13 letter word FET(E) CH,A,M(P)ETRE
F
E
T
(
E
)
C
H
,
A
,
M
(
P
)
E
T
R
E
Buddy explains!
The straight part
We know that "Garden party" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case garden party = fet(e) ch,a,m(p)etre.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.