Responsibility of ordinary group of students (4)
I believe the answer is:
onus
'responsibility' is the definition.
(onus is a duty or responsibility)
'ordinary group of students' is the wordplay.
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'group of students' becomes 'NUS' (National Union of Students).
'o'+'nus'='ONUS'
'of' is the link.
(Other definitions for onus that I've seen before include "Encumbrance" , "Responsibilty" , "Capulet opponent (R&J)" , "Duty or responsibility" , "Liability" .)