Wonder how expert's homework attains low grade? (7)
I believe the answer is:
prodigy
'wonder' is the definition.
(thesaurus)
'expert's homework attains low grade?' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'homework' becomes 'diy' (DIY is work done on your home).
'attains' is an insertion indicator.
'low grade?' becomes 'g' (low grade in some exams).
'diy' enclosing 'g' is 'digy'.
'pro'+'digy'='PRODIGY'
'how' is the link.
(Other definitions for prodigy that I've seen before include "Amazingly talented child" , "Amazing thing" , "Child genius" , "Youngster of exceptional abilities" .)