The most common and most likely answer to this crossword clue is the 7 letter word SIAMESE
I've seen this crossword clue in Independent
S
I
A
M
E
S
E
More clues leading to the result SIAMESE
- Answer is to go up with string to join pipes the old way
- Sort of cat, one housed in identical quarters
- Sort of cat is returned in the morning and leashed regularly
- Cat sees bats about one in the morning
- Uniform worn by one extremely statuesque queen?
- Tripping? As I seem to be a cat
- Some split hairs to update this language
- National sees I am struggling
- Some split hairs to modernise this Asian language
- Not all of these maisonettes rejected cat
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 = siamese.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.