The most common and most likely answer to this crossword clue is the 13 letter word INDISPENSABLY
I've seen this crossword clue in Independent
I
N
D
I
S
P
E
N
S
A
B
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Buddy explains!
The straight part
We know that "as /an absolute necessity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as /an absolute necessity = indispensably.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.