The most common and most likely answer to this crossword clue is the 5 letter word QUARK
I've seen this crossword clue in Financial Times
Q
U
A
R
K
Buddy explains!

The straight part
We know that "Particle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case particle = quark.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 QUARK mean?
QUARK noun- In the Standard Model, an elementary subatomic particle that forms matter. They combine to form hadrons, such as protons and neutrons.
- (X Window System) An integer that uniquely identifies a text string.
- A soft creamy cheese, eaten throughout northern, central, and eastern Europe, very similar to cottage cheese except that it is usually not made with rennet.
- (Falkland Islands) The black-crowned night heron, Nycticorax nycticorax.