Crossword Genius

Teacher in old college wasting time (3)

Ross

I believe the answer is:

sir

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'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" .)

I've seen this clue in The Times and The Sunday Times.
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