Brain cell (6)
I believe the answer is:
neuron
'brain cell' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for neuron that I've seen before include "Nerve impulse cell" , "Transmitting nerve cell" , "It transmits nerve impulses" , "Impulsive thing" , "Cell that conducts nerve impulses" .)