Responsibility of old group of students (4)
I believe the answer is:
onus
'responsibility' is the definition.
(onus is a duty or responsibility)
'old group of students' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'group of students' becomes 'NUS' (National Union of Students).
'o'+'nus'='ONUS'
'of' acts as a link.
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Capulet opponent (R&J)" , "Encumbrance" , "Duty or responsibility" , "Burden, duty" .)