The most common and most likely answer to this crossword clue is the 3 plus 2 plus 1 plus 2 letter word BIT OF A DO
I've seen this crossword clue in Financial Times
B
I
T
O
F
A
D
O
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 = bit of a do.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.