Endlessly insolent boy is individual (6)
I believe the answer is:
person
'is individual' is the definition.
(an individual person)
'endlessly insolent boy' is the wordplay.
'endlessly insolent' becomes 'per' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boy' becomes 'son' (male child).
'per'+'son'='PERSON'
Can you help me to learn more?
(Other definitions for person that I've seen before include "Bod" , "character" , "A man or woman" , "present appearing in this" , "he's the third" .)