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 judge whether it can be defined by this definition.
'see the old repeatedly accepting unknown' is the wordplay.
'see' becomes 'la' (I am not sure about this - if you are sure you should believe this answer much more).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'repeatedly' shows that letters should be duplicated.
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'unknown' becomes 'z' (algebra).
'ye' duplicated 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" .)