The most common and most likely answer to this crossword clue is the 7 plus 8 letter word GENERAL ELECTION
I've seen this crossword clue in The Daily Telegraph
G
E
N
E
R
A
L
E
L
E
C
T
I
O
N
More clues leading to the result GENERAL ELECTION
Buddy explains!
The straight part
We know that "Event requiring national crosses?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case event requiring national crosses? = general election.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.