The most common and most likely answer to this crossword clue is the 12 letter word ANTIPARTICLE
I've seen this crossword clue in The Times
A
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Buddy explains!
The straight part
We know that "Some unusual matter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case some unusual matter = antiparticle.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 ANTIPARTICLE mean?
ANTIPARTICLE noun- A subatomic particle corresponding to another particle with the same mass, spin and mean lifetime but with charge, parity, strangeness and other quantum numbers flipped in sign; a particle that has a reversed world line to another.