Teacher, possibly, avoiding time in jail (3)

I believe the answer is:

sir

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'teacher possibly' is the definition.
(a male teacher might be addressed as 'sir')

I cannot understand how the rest of the clue works.

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(Other definitions for sir that I've seen before include "Polite form of address to a man" , "... ___ John Major" , "Title of 1 across" , "Feted fellow" , "Term of address for a gentleman" .)

I've seen this clue in The Times.

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