Light-sensitive membrane (6)
I believe the answer is:
retina
'light-sensitive membrane' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for retina that I've seen before include "Ocular membrane" , "What's behind the humour" , "Membrane covering the back of the eyeball" , "Sensitive layer in eye" , "Eye's membrane" .)