The most common and most likely answer to this crossword clue is the 9 letter word INSTALLED
I've seen this crossword clue in The Guardian
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More clues leading to the result INSTALLED
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 = installed.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 INSTALLED mean?
INSTALLED verb- To connect, set up or prepare something for use.
- To admit formally into an office, rank or position.
- To establish or settle in.