The most common and most likely answer to this crossword clue is the 9 plus 3 letter word SELF-MADE MAN
I've seen this crossword clue in Financial Times
S
E
L
F
-
M
A
D
E
M
A
N
Buddy explains!
The straight part
We know that "arriviste" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case arriviste = self-made man.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.