Item that is charged having been chosen and not put back (8)
I believe the answer is:
electron
'item that is charged having been' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'chosen and not put back' is the wordplay.
'chosen' becomes 'elect' ('elect' as an adjective can mean chosen).
'and not' becomes 'nor' ('nor' means 'and also not').
'put back' shows that the letters should be reversed in order.
'nor' reversed gives 'ron'.
'elect'+'ron'='ELECTRON'
Can you help me to learn more?
(Other definitions for electron that I've seen before include "Small matter" , "One's charged" , "There's not much to it" , "Charged subatomic particle" , "Negatively charged particle" .)