Some decor nearly forms covering of eyeball (6)
I believe the answer is:
cornea
'covering of eyeball' is the definition.
(I've seen this before)
'some decor nearly forms' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'CORNEA' is hidden within 'decor nearly forms'.
(Other definitions for cornea that I've seen before include "Transparent part of the coat of the eyeball" , "Membrane of the eye" , "Transparent part of the eye" , "Outer covering of the eye" , "bit of a looker" .)