Turn one off a particle (7)
I believe the answer is:
neutron
'a particle' is the definition.
(type of sub-atomic particle)
'turn one off' is the wordplay.
'off' indicates anagramming the letters.
'turn'+'one'='turnone'
'turnone' anagrammed gives 'NEUTRON'.
(Other definitions for neutron that I've seen before include "Subatomic particle with no electric charge" , "one's not charged" , "Turn one (anag) - kind of bomb" , "On tuner (anag.)" , "Elementary particle with zero charge" .)