The most common and most likely answer to this crossword clue is the 12 letter word INVALIDATION
I
N
V
A
L
I
D
A
T
I
O
N
Buddy explains!
The straight part
We know that "it will make it null and void" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it will make it null and void = invalidation.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.