The most common and most likely answer to this crossword clue is the 5 plus 4 letter word PHNOM PENH
P
H
N
O
M
P
E
N
H
Buddy explains!

The straight part
We know that "Cambodian capital" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cambodian capital = phnom penh.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.