The most common and most likely answer to this crossword clue is the 8 plus 3 letter word WAITANGI DAY
I've seen this crossword clue in The Times
W
A
I
T
A
N
G
I
D
A
Y
Buddy explains!
The straight part
We know that "Feb 6 NZ celebration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case feb 6 nz celebration = waitangi day.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.