The most common and most likely answer to this crossword clue is the 6 plus 1 plus 6 letter word PRAGUE / PLAGUE
I've seen this crossword clue in Financial Times
P
R
A
G
U
E
/
P
L
A
G
U
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 = prague / plague.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.