Popular group of students rejected believer (5)
I believe the answer is:
sunni
'believer' is the definition.
(a Sunni Muslim)
'popular group of students rejected' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'group of students' becomes 'NUS' (National Union of Students).
'rejected' says the letters should be written backwards.
'in'+'nus'='innus'
'innus' written backwards gives 'SUNNI'.
(Other definitions for sunni that I've seen before include "Indian fig tree with aerial roots" , "A main branch of Islam" , "Form of religion" , "Islamic faction" , "Moslem" .)