Nerve impulse carrier (4)
I believe the answer is:
axon
'nerve impulse carrier' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for axon that I've seen before include "Part of cell transmitting" , "impulsive conductor" , "Bit of a nerve!" , "Disabling headache" , "extension in cell" .)