Acre on settlement is in front of one's eye (6)
I believe the answer is:
cornea
'is in front of one's eye' is the definition.
'acre on settlement' is the wordplay.
'settlement' indicates anagramming the letters.
'acre'+'on'='acreon'
'acreon' with letters rearranged gives 'CORNEA'.
(Other definitions for cornea that I've seen before include "It covers the eye" , "Clear front layer of eye" , "Eyeball membrane" , "Transparent covering of the eye" , "bit of a looker" .)