The most common and most likely answer to this crossword clue is the 5 plus 10 letter word BRAND AMBASSADOR
B
R
A
N
D
A
M
B
A
S
S
A
D
O
R
Buddy explains!
The straight part
We know that "the promoter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the promoter = brand ambassador.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.