Muslim belonging to a student group rejected (5)
I believe the answer is:
sunni
'muslim' is the definition.
(I know that Sunni is a type of muslim)
'belonging to a student group rejected' is the wordplay.
'belonging to' becomes 'in' (in can mean being a member of).
'a student group' becomes 'NUS' (National Union of Students).
'rejected' says the letters should be written backwards.
'in'+'nus'='innus'
'innus' backwards is 'SUNNI'.
(Other definitions for sunni that I've seen before include "Form of religion" , "Moslem" , "One branch of Islam" , "Member of another Muslim sect" , "Indian fig tree with aerial roots" .)