The most common and most likely answer to this crossword clue is the 6 letter word EXTERN
I've seen this crossword clue in The Daily Telegraph
E
X
T
E
R
N
Buddy explains!

The straight part
We know that "non-resident" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case non-resident = extern.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 EXTERN mean?
EXTERN noun- A person affiliated with an institution in a lesser capacity, for example, as a non-resident or as a part-time affiliate.
- Outward form or part; exterior.
- External; outward; not inherent
- In the C and C++ programming languages, a variable that can be separately declared in many places, all of them referring to the same variable.