Iris layer (4)
I believe the answer is:
uvea
'iris layer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for uvea that I've seen before include "Part of the eyeball which houses the iris" , "bit of a looker" , "part of eye be seen" , "Pigmented part of eye" , "The pigmented layer of the eye" .)