A lecturer at college retains marks for former students (6)
I believe the answer is:
alumni
'former students' is the definition.
(I've seen this before)
'a lecturer at college retains marks' is the wordplay.
'lecturer' becomes 'l' (abbreviation for lecturer).
'at' says to put letters next to each other.
'college' becomes 'uni' (short for university).
'retains' indicates putting letters inside.
'marks' becomes 'M' (former German currency).
'uni' enclosing 'm' is 'umni'.
'a'+'l'+'umni'='ALUMNI'
'for' is the link.
(Other definitions for alumni that I've seen before include "Ex-students" , "Former (school) pupils" , "Past pupils, Latin plural" , "Graduates of a college" , "Graduates of an institution" .)