The most common and most likely answer to this crossword clue is the 11 letter word BANGLADESHI
I've seen this crossword clue in The Times
B
A
N
G
L
A
D
E
S
H
I
More clues leading to the result BANGLADESHI
Buddy explains!

The straight part
We know that "National" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case national = bangladeshi.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.