The most common and most likely answer to this crossword clue is the 3 plus 6 letter word ART DEALER
I've seen this crossword clue in Financial Times
A
R
T
D
E
A
L
E
R
More clues leading to the result ART DEALER
Buddy explains!
The straight part
We know that "Oil trader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oil trader = art dealer.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.