One upset teacher means to cut pupil down to size! (4)
I believe the answer is:
iris
'to cut pupil down to size' is the definition.
I can't tell whether this definition defines the answer.
'one upset teacher' is the wordplay.
'one' becomes 'i' (Roman numeral).
'upset' says the letters should be written backwards.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'sir' back-to-front is 'ris'.
'i'+'ris'='IRIS'
'means' is the link.
Can you help me to learn more?
(Other definitions for iris that I've seen before include "Nice flower or eye part" , "One controlling pupil' s reactions" , "Bright flower or eye part" , "Pupil's place?" , "A flower; spirit conjured in Tempest" .)