The most common and most likely answer to this crossword clue is the 11 letter word COORDINATOR
I've seen this crossword clue in Financial Times
C
O
O
R
D
I
N
A
T
O
R
Buddy explains!

The straight part
We know that "One making arrangements" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one making arrangements = coordinator.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 COORDINATOR mean?
COORDINATOR noun- One who coordinates.
- An assistant coach responsible for a particular facet of the game, such as defense.
- (grammar) A member of a lexical class of words that joins words, phrases, and clauses at the same syntactic level.