The most common and most likely answer to this crossword clue is the 6 letter word COHOST
I've seen this crossword clue in The Times
C
O
H
O
S
T
Buddy explains!

The straight part
We know that "party organiser (not sole)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case party organiser (not sole) = cohost.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 COHOST mean?
COHOST noun- A joint host alongside another (compare costar).
- To act as a joint host.
- To store data or applications on a shared server (as in web hosting).