The most common and most likely answer to this crossword clue is the 9 letter word VIENTIANE
I've seen this crossword clue in The Guardian
V
I
E
N
T
I
A
N
E
More clues leading to the result VIENTIANE
Buddy explains!

The straight part
We know that "Asian capital" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case asian capital = vientiane.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.