The most common and most likely answer to this crossword clue is the 7 letter word BAGHDAD
I've seen this crossword clue in Independent
B
A
G
H
D
A
D
More clues leading to the result BAGHDAD
- Capital needed, get old man to invest cash, finally
- Swinging handbag, daughter knocks out penultimate member of gang in foreign capital
- Father under secure hospital in the city
- Catch PA stealing bit of hard-earned capital
- City needs a good hospital — 500 coming in sick
- Speciality shown by husband and father in ME city
- Had bad shivering around midnight in capital city
- Secure father outside hospital in capital city
- Place in Iraq caught commercial's sound
- Land hot papa with foreign capital
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 = baghdad.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.