Endless slander I attributed to old girls and boys (6)
I believe the answer is:
alumni
'to old girls and boys' is the definition.
(graduates of a school or university)
'endless slander i' is the wordplay.
'endless' means to look at the middle letters.
'slander' becomes 'calumny' (synonyms).
The middle of 'calumny' is 'alumn'.
'alumn'+'i'='ALUMNI'
'attributed' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for alumni that I've seen before include "Past pupils, former students" , "Student population once" , "Put too much strain on, as Revenue Commissioners do" , "Old boys and girls" , "Past pupils of the old school" .)