Neuron part (4)
I believe the answer is:
axon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">neuron part</span>' is the definition.<br/ ><span class="explanation-format__explanation">('axon' is part of 'neuron')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for axon that I've seen before include "Disabling headache" , "extension in cell" , "Bit of a nerve!" , "Part of cell transmitting" , "impulsive conductor" .)