The most common and most likely answer to this crossword clue is the 5 plus 5 letter word BRAIN DRAIN
B
R
A
I
N
D
R
A
I
N
More clues leading to the result BRAIN DRAIN
- Head of Balliol departs in lots of wet weather — a loss of talent
- Shortage of decent cells with migrants slowly escaping?
- Emigration supporter popular with Democrat has a downfall
- Cure for hydrocephalus leads to loss of intelligence
- Support independent artist, one with new ongoing loss of talent?
- Learned moves from bachelor, finally honed in showers
- Intelligence leak
- Daughter, caught between showers east of Bangladeshi capital, minds going abroad?
- Cause of diminishing intelligence
- Bob’s first and David’s last to lead precipitation of intellectual loss
Buddy explains!

The straight part
We know that "Nations’ loss of talent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nations’ loss of talent = brain drain.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.