Smart Alec's brother? One is irritatingly knowledgeable (6,4)
I believe the answer is:
clever dick
'brother? one is irritatingly knowledgeable' is the definition.
I can't judge whether this definition defines the answer.
'smart alec's' is the wordplay.
'smart' becomes 'clever' (similar in meaning).
'alec's' becomes 'dick' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'clever'+'dick'='CLEVER DICK'
Can you help me to learn more?
(Other definitions for clever dick that I've seen before include "The one with all the answers" , "Wise guy" , "Know-all" .)