The most common and most likely answer to this crossword clue is the 6 letter word EXPATS
I've seen this crossword clue in The Daily Telegraph
E
X
P
A
T
S
More clues leading to the result EXPATS
Buddy explains!

The straight part
We know that "Nationals abroad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nationals abroad = expats.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 EXPATS mean?
EXPATS noun- An expatriate; a person who lives outside their own country.