The most common and most likely answer to this crossword clue is the 5 letter word VITAL
I've seen this crossword clue in Independent
V
I
T
A
L
More clues leading to the result VITAL
- Little Valerie describing information technology as crucial
- Key — it opens throne room from the back
- Key discussion curtailed after six
- What’s essential — some speculative returns
- Girl maintaining it’s indispensable
- Key component of speculative returns
- Temperature in medicine bottle’s key
- Group of relatives circulate key
- Group of relatives returned key
- Extremely important Russian not unknown
Buddy explains!

The straight part
We know that "Extremely important" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely important = vital.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 VITAL mean?
VITAL adjective- Relating to, or characteristic of life.
- Necessary to the continuation of life; being the seat of life; being that on which life depends.
- Invigorating or life-giving.
- Necessary to continued existence.
- Relating to the recording of life events.
- Very important.
- Containing life; living.
- Capable of living; in a state to live; viable.