The most common and most likely answer to this crossword clue is the 3 plus 3 letter word GET OUT
I've seen this crossword clue in The Daily Telegraph
G
E
T
O
U
T
Buddy explains!
The straight part
We know that "Expulsion order" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expulsion order = get out.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 GET OUT mean?
GET OUT noun- An opportunity to escape from an undesirable situation, such as a legal loophole.
- The dismantling and removal of equipment, scenery, etc. when leaving a theater or television studio at the end of a production.