It's atomic, as in nuclear fission (3)

I believe the answer is:

ion

I'm a little stuck... Click here to teach me more about this clue!

'it's atomic as' is the definition.
I can't tell whether this defines the answer.

'nuclear fission' is the wordplay.
'ION' is hidden in amongst the letters.
But, I'm uncertain how this is indicated.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for ion that I've seen before include "Electrically-charged particle" , "Charged atom/molecule" , "Eletrically charged particle" , "Subatomic particle" , "Particle (phys.)" .)

I've seen this clue in the Evening Standard.

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