Teacher is turning right (3)
I believe the answer is:
sir
'teacher' is the definition.
(I've seen this before)
'is turning right' is the wordplay.
'turning' is a reversal indicator.
'right' becomes 'r' (common abbreviation).
'is' written backwards gives '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" .)