The most common and most likely answer to this crossword clue is the 9 letter word CONFIGURE
I've seen this crossword clue in The Daily Telegraph
C
O
N
F
I
G
U
R
E
Buddy explains!
The straight part
We know that "Set up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case set up = configure.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 CONFIGURE mean?
CONFIGURE verb- To set up or arrange something in such a way that it is ready for operation for a particular purpose, or to someone's particular liking