Male teacher is up on top of roof (3)
I believe the answer is:
sir
'male teacher' is the definition.
(traditional term of address for a male teacher)
'is up on top of roof' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on top of' says to take the initial letters.
The first letter of 'roof' is 'r'.
'is' in reverse letter order is 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Gentleman's form of address" , "Knight's title" , "Polite term of address for a man" , "Title of 1 across" , "Polite or formal address to male" .)