One teacher put up a flower (4)
I believe the answer is:
iris
'a flower' is the definition.
(thesaurus)
'one teacher put up' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'teacher' becomes 'sir' (I've seen this before).
'put up' is a reversal indicator.
'sir' back-to-front is 'ris'.
'i'+'ris'='IRIS'
(Other definitions for iris that I've seen before include "Pupil's place?" , "Nice flower or eye part" , "is 21[WOMAN], perhaps" , "Pupil is at the centre of this" , "Coloured part of the lamp" .)