The most common and most likely answer to this crossword clue is the 8 plus 2 plus 5 letter word SANTIAGO DE CHILE
I've seen this crossword clue in Financial Times
S
A
N
T
I
A
G
O
D
E
C
H
I
L
E
Buddy explains!
The straight part
We know that "Capital" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case capital = santiago de chile.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.