Relating to the nervous system (6)
I believe the answer is:
neural
'relating to the nervous system' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for neural that I've seen before include "Relating to a nerve" , "Relating to the nervous system" , "To do with nerve of unreal kind" , "Nervy" , "Relating to nerves" .)