The most common and most likely answer to this crossword clue is the 13 letter word INDISPENSABLE
I've seen this crossword clue in Independent
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More clues leading to the result INDISPENSABLE
- Popular plan, besides, crazy but essential
- Popular writer’s wearing fur after initially deeming it essential
- Essential corruption of a penniless bid
- Hell to write, breaking through dark as necessary
- Vital spies blend in, adapting around America
- Get painless in bed somehow — that’s essential
- Key spies blend in somehow, casing area
- … adapting pine bed’s nails to make key
- Essential spaniel’s in bed after going walkies
- Vital hip damage straddling fences
Buddy explains!

The straight part
We know that "essential" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case essential = indispensable.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 INDISPENSABLE mean?
INDISPENSABLE noun- A thing that is not dispensable; a necessity.
- (in the plural) Trousers.
- Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
- (of duties, rules etc.) Unbendable, that cannot be set aside or ignored.
- Absolutely necessary or requisite; that one cannot do without.