Outer layer of the eyeball (6)
I believe the answer is:
sclera
'outer layer of the eyeball' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sclera that I've seen before include "tissue in the eye" , "Bit of a looker" , "Membrane" , "Eyeball's outer cover" , "part of the eye" .)