Particle beginning to trigger inside nerve cell (7)
I believe the answer is:
neutron
'particle' is the definition.
(type of sub-atomic particle)
'beginning to trigger inside nerve cell' is the wordplay.
'beginning to' suggests taking the first letters.
'inside' indicates putting letters inside.
'nerve cell' becomes 'neuron' (synonyms).
The initial letter of 'trigger' is 't'.
't' going into 'neuron' is 'NEUTRON'.
(Other definitions for neutron that I've seen before include "Elementary particle with zero charge" , "Chargeless particle" , "Charged particle" , "On tuner (anag.)" , "Particle that decays when released" .)