The most common and most likely answer to this crossword clue is the 11 letter word INTERCESSOR
I've seen this crossword clue in The Guardian
I
N
T
E
R
C
E
S
S
O
R
Buddy explains!

The straight part
We know that "Negotiator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case negotiator = intercessor.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 INTERCESSOR mean?
INTERCESSOR noun- A person who intercedes; a mediator; one who reconciles enemies, or pleads for another.
- A bishop who acts during a vacancy in a see.