See the old, repeatedly accepting unknown visual defect (4,3)
I believe the answer is:
lazy eye
'visual defect' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'see the old repeatedly accepting unknown' is the wordplay.
'see' becomes 'la' (I can't justify this - if you can you should believe this answer much more).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'repeatedly' indicates the doubling of some letters.
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'unknown' becomes 'z' (represents an unknown value in algebra).
'ye' doubled is 'yeye'.
'la'+'yeye'='layeye'
'layeye' enclosing 'z' is 'LAZY EYE'.
Can you help me to learn more?
(Other definitions for lazy eye that I've seen before include "this globe doesn't turn in its orbit" , "An apparently healthy but impaired looker?" , "Optical problem" , "Visual impairment with no sign of damage" .)