Teacher in old college wasting time (3)
I believe the answer is:
sir
'teacher' is the definition.
(a male teacher might be addressed as 'sir')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Term of address for a gentleman" , "Honorary male title" , "Respectful address for a man" , "address to teacher" , "Poet and novelist" .)