Sporty student to initiate classy descent? (4,5)
I believe the answer is:
blue blood
'classy descent?' is the definition.
(descent from the upper classes)
'sporty student to initiate' is the wordplay.
'sporty student' becomes 'blue' (blue can mean an Oxford or Cambridge sportsperson).
'to initiate' becomes 'blood' (blood can mean to initiate someone in an activity).
'blue'+'blood'='BLUE BLOOD'
(Other definitions for blue blood that I've seen before include "Aristocratic descent" , "What circulates in aristocratic circles?" .)