A little matter (4)
'a little matter' is the definition.
(tiny particle of matter)
This is the entire clue.
(Other definitions for atom that I've seen before include "Small amount of matter" , "An explosive particle as in bomb" , "Nucleus surrounded" , "A small but mighty particle" , "It was once thought to be smallest particle of matter" .)
This could be a double definition.
'a little' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'matter' is the second definition.
(ion is a kind of matter)
Can you help me to learn more?
(Other definitions for ion that I've seen before include "Charged atom/molecule" , "Eletrically charged particle" , "Particle (phys.)" , "Electrically charged particle" , "Electrically-charged particle" .)