The most common and most likely answer to this crossword clue is the 13 letter word HOUSE-WARMING
I've seen this crossword clue in The Guardian
H
O
U
S
E
-
W
A
R
M
I
N
G
More clues leading to the result HOUSE-WARMING
Buddy explains!
The straight part
We know that "Party" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case party = house-warming.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.
What does HOUSE-WARMING mean?
HOUSE-WARMING noun- A party to celebrate moving into a new home.
- The act of welcoming a person/family to their newly purchased or newly rented home.