Idiots' son represented lacking adult opinion (10)
I believe the answer is:
assessment
'opinion' is the definition.
(I've seen this before)
'idiots' son represented lacking adult' is the wordplay.
'idiots'' becomes 'asses' (I've seen this before).
'son' becomes 's' (genealogical abbreviation for son).
'represented lacking adult' becomes 'ment' (I am not sure about this - if you are sure you should believe this answer much more).
'asses'+'s'+'ment'='ASSESSMENT'
Can you help me to learn more?
(Other definitions for assessment that I've seen before include "Survey" , "Review" , "Examination" , "Opinion" , "Appraisal" .)