The most common and most likely answer to this crossword clue is the 7 letter word VIOLENT
I've seen this crossword clue in The Guardian
V
I
O
L
E
N
T
More clues leading to the result VIOLENT
- Aggressive workman finally admitted to plant
- Fierce girl enthralling any number
- Frenzied sex over before period of abstinence
- Numbers five and ten fast and aggressive
- Intense colour contains navy
- Wild flower absorbing nitrogen
- Wild flower containing nectar, primarily
- Not evil — maybe just lacking restraint
- Extreme north shrouded in reddish-blue
- Furious cook lit oven
Buddy explains!

The straight part
We know that "Extreme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extreme = violent.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 VIOLENT mean?
VIOLENT noun- An assailant.
- To urge with violence.
- Involving extreme force or motion.
- Involving physical conflict.
- Likely to use physical force.
- Intensely vivid.
- Produced or effected by force; not spontaneous; unnatural.