The most common and most likely answer to this crossword clue is the 5 plus 4 letter word THROW DOWN
I've seen this crossword clue in The Guardian
T
H
R
O
W
D
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W
N
Buddy explains!

The straight part
We know that "Demolish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demolish = throw down.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 THROW DOWN mean?
THROW DOWN noun- A fight or brawl.
- A challenge or incitement to fight.
- A weapon planted at a crime scene in order to mislead investigators.