Uncharged atomic particle (7)
I believe the answer is:
neutron
'uncharged atomic particle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for neutron that I've seen before include "Subatomic particle with no electric charge" , "Chargeless particle" , "Uncharged atomic particle" , "one isn't charged" , "Uncharged subatomic particle" .)