OK to enter figure, seeing ability (8)
I believe the answer is:
eyesight
'seeing ability' is the definition.
(I've seen this before)
'ok to enter figure' is the wordplay.
'ok' becomes 'yes' (I've seen this before).
'to enter' is an insertion indicator.
'figure' becomes 'eight' (I've seen this in other clues).
'yes' placed within 'eight' is 'EYESIGHT'.
(Other definitions for eyesight that I've seen before include "Visual faculty" , "Faculty of vision" , "Perceptive faculty" , "Ability to see" , "What's needed to see" .)