The most common and most likely answer to this crossword clue is the 9 letter word PUNCHIEST
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "extremely hard-hitting" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely hard-hitting = punchiest.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 PUNCHIEST mean?
PUNCHIEST adjective- Having a punch; effective; forceful; spirited; vigorous.
- Behaving or appearing punch drunk.
- (of a person) Being over-reactive to routine events.
- A term used by Nordic skiers (especially skate skiers) to describe groomed snow that does not support the weight of a skier, especially when the skier’s weight is all on one ski, resulting in a ski punching through the surface of the snow.
- Short and thick; fat; paunchy