Teacher is backward at any one of the three Rs! (3)

I believe the answer is:

sir

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

I don't understand how the rest of the clue works.

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(Other definitions for sir that I've seen before include "Knight's title" , "Honorary male title" , "Title of 1 across" , "Title given to 10 Across" , "Title given to baronet" .)

I've seen this clue in the Evening Standard.

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