Charged particle (3)
'charged particle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ion that I've seen before include "Electrically charged particle" , "Subatomic particle" , "Charged atom/molecule" , "Electrically charged atom or group of atoms" , "Particle (phys.)" .)
'charged particle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for electron that I've seen before include "Particle found in all atoms" , "Subatomic particle -- enrol etc (anag)" , "Tiny bit of power" , "Charged subatomic particle" , "There's not much to it" .)