The most common and most likely answer to this crossword clue is the 6 letter word GERMAN
I've seen this crossword clue in Financial Times
G
E
R
M
A
N
More clues leading to the result GERMAN
- Not entirely relevant language
- Ranger managed to arrest European
- Hamburger, say, as part of hunger management?
- Organism spreading disease discovered by a north European
- National explosion of anger about opening of motorway
- Language from harassed manager, having lost heart?
- Beginning article in English or other language
- Dope grasps reading mother tongue
- Briefly relevant to European
- European
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 = german.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.
What does GERMAN mean?
GERMAN noun- A near relative.
- Having the same mother and father; a full (brother or sister).
- Being born to one’s blood aunt or uncle, a first (cousin).
- Closely related, akin.
- An elaborate round dance, often with a waltz movement.
- A social party at which the german is danced.