Private investigator in unruly Mayo seeing problem (6)
I believe the answer is:
myopia
'seeing problem' is the definition.
(I've seen this before)
'private investigator in unruly mayo' is the wordplay.
'private investigator' becomes 'PI' (abbreviation).
'in' means one lot of letters goes inside another.
'unruly' is an anagram indicator.
'mayo' with letters rearranged gives 'myoa'.
'pi' inserted within 'myoa' is 'MYOPIA'.
(Other definitions for myopia that I've seen before include "Lack of insight" , "lack of long-term vision" , "complaint" , "Condition of short sightedness" , "Seeing problem" .)