Teacher is turning right (3)
I believe the answer is:
sir
'teacher' is the definition.
(a male teacher might be addressed as 'sir')
'is turning right' is the wordplay.
'turning' says the letters should be written in reverse.
'right' becomes 'r' (common abbreviation).
'is' back-to-front is 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Baronet's title" , "Honorary male title" , "Knight's prefix" , "Title of 1 across" , "Way to address gentleman" .)