Only externally visible part of the central nervous system (6)
I believe the answer is:
retina
'only externally visible part of the central nervous system' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for retina that I've seen before include "What's behind the humour" , "Light-sensitive eye layer" , "Light-receptive part of eye" , "Membrane covering the back of the eyeball" , "Light-sensitive membrane in the eye" .)