Of the nervous system (6)
I believe the answer is:
neural
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">of the nervous system</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for neural that I've seen before include "Relating to the nervous system" , "Nervy" , "Relating to nerves" , "To do with nerve of unreal kind" , "Relating to a nerve" .)