The most common and most likely answer to this crossword clue is the 9 plus 7 letter word CHARACTER WITNESS
I've seen this crossword clue in The Times
C
H
A
R
A
C
T
E
R
W
I
T
N
E
S
S
Buddy explains!
The straight part
We know that "Participant in trial" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case participant in trial = character witness.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.