Advantages of good grades put before classes (6)
I believe the answer is:
assets
'advantages' is the definition.
(an asset is something advantageous)
'good grades put before classes' is the wordplay.
'good grades' becomes 'A's' (A grades).
'put before' says to put letters next to each other.
'classes' becomes 'sets' (I've seen this before).
'as'+'sets'='ASSETS'
'of' acts as a link.
(Other definitions for assets that I've seen before include "Useful or valuable things" , "worldly goods" , "(Person's) property" , "Company's property" , "capital" .)