The most common and most likely answer to this crossword clue is the 9 letter word REQUISITE
R
E
Q
U
I
S
I
T
E
More clues leading to the result REQUISITE
Buddy explains!

The straight part
We know that "Necessary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case necessary = requisite.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 REQUISITE mean?
REQUISITE noun- An indispensable item; a requirement.
- Essential, indispensable, required.