Temperature inside nerve cell particle (7)
I believe the answer is:
neutron
'particle' is the definition.
(type of sub-atomic particle)
'temperature inside nerve cell' is the wordplay.
'temperature' becomes 't' (abbreviation).
'inside' indicates putting letters inside.
'nerve cell' becomes 'neuron' (synonyms).
't' going into 'neuron' is 'NEUTRON'.
(Other definitions for neutron that I've seen before include "Subatomic particle with no electric charge" , "Chargeless particle" , "On tuner (anag.)" , "one's not charged" , "Uncharged particle in a nucleus" .)