The most common and most likely answer to this crossword clue is the 14 letter word NATURALISATION
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "acquiring citizenship of another country" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case acquiring citizenship of another country = naturalisation.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 NATURALISATION mean?
NATURALISATION noun- The action of naturalizing somebody; act of granting citizenship.
- The admission or adoption of foreign words or customs into general use.
- The introduction and establishment of an animal or plant into a place where it is not indigenous.