Rebellious at university, elected members (2,2,4)
I believe the answer is:
up in arms
'rebellious' is the definition.
(thesaurus)
'at university elected members' is the wordplay.
'at university' becomes 'up'.
'elected' becomes 'in'.
'members' becomes 'arms' (arm is a kind of member).
'up'+'in'+'arms'='UP IN ARMS'
(Other definitions for up in arms that I've seen before include "Openly protesting" , "Young child may be found here" , "Protesting vigorously, in militant mood" , "Better prepared for battle" , "Protesting vigorously, ready for fight" .)