The most common and most likely answer to this crossword clue is the 11 letter word FIRST-PARTY
I've seen this crossword clue in The Guardian
F
I
R
S
T
-
P
A
R
T
Y
Buddy explains!
The straight part
We know that "someone directly involved" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case someone directly involved = first-party.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.