Teacher I stood up, bit of a looker? (4)
I believe the answer is:
iris
'bit of a looker?' is the definition.
(I've seen this before)
'teacher i stood up' is the wordplay.
'teacher' becomes 'sir' (way of addressing a male teacher).
'stood up' says the letters should be written backwards (in a down clue, letters go up).
'sir'+'i'='siri'
'siri' written backwards gives 'IRIS'.
(Other definitions for iris that I've seen before include "Part of the eye, usually brown or blue" , "In the middle of which pupil is opening" , "Garden flower - part of eye" , "is 21[WOMAN], perhaps" , "A flower; spirit conjured in Tempest" .)