Teacher, possibly, avoiding time in jail (3)
I believe the answer is:
'teacher possibly' is the definition.
(a male teacher might be addressed as 'sir')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(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" .)