Of the nervous system (6)
I believe the answer is:
neural
'of the nervous system' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)