The most common and most likely answer to this crossword clue is the 6 letter word BARYON
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The straight part
We know that "atomic particle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case atomic particle = baryon.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 BARYON mean?
BARYON noun- A heavy subatomic particle created by the binding of quarks by gluons; a hadron containing three quarks. Baryons have half-odd integral spin and are thus fermions. This category includes the common proton and neutron of the atomic nucleus.