Crossword Genius

Both kings out of joint regarding very small thing (7)

Ross

I believe the answer is:

nucleon

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'very small thing' is the definition.
(subatomic particle)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nucleon that I've seen before include "fundamental constituent" , "Particle" .)

I've seen this clue in The Times.
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