Teacher is tripping over end of ruler (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'is tripping over end of ruler' is the wordplay.
'tripping' indicates anagramming the letters.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'end of' says to take the final letters.
The last letter of 'ruler' is 'r'.
'is' anagrammed gives 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Gentleman's form of address" , "Way to address gentleman" , "Title given to baronet" , "Feted fellow" , "Honorary male title" .)