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