Crossword Genius

Male teacher is up on top of roof (3)

Ross

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" .)

I've seen this clue in The Telegraph.
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