The most common and most likely answer to this crossword clue is the 6 letter word ÉMIGRÉ
I've seen this crossword clue in The Guardian
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More clues leading to the result ÉMIGRÉ
- One who’s fled bad regime?
- One leaving country before tackling fighter
- Exile foreign ruler possessing good English
- Outgoing type of regular mermaid girl, eh?
- Victim of regime change?
- Label King George put on European refugee
- He leaves country before boxing fighter
- The Comte d’Artois was one for a change of regime
- Product of an evil regime?
- One fleeing Iraq, perhaps after regime change
Buddy explains!

The straight part
We know that "politically motivated leaver" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case politically motivated leaver = ÉmigrÉ.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 ÉMIGRÉ mean?
ÉMIGRÉ noun- One who has departed their native land, often as a refugee.
- An emigrant, one who departs their native land to become an immigrant in another.