Some calumnious old students (6)
I believe the answer is:
alumni
'students' is the definition.
(alumnus is a kind of student)
'some calumnious old' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ALUMNI' is hidden in the letters of 'calumnious old'.
(Other definitions for alumni that I've seen before include "Put too much strain on, as Revenue Commissioners do" , "Past pupils, former students" , "Graduates of a college" , "Past pupils of the old school" , "Former (college) students" .)