Girl puts one over teacher being sent up (4)
I believe the answer is:
iris
'girl' is the definition.
'one over teacher being sent up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'over' means one lot of letters go next to another.
'teacher' becomes 'sir' (I've seen this before**).
'being sent up' shows that the letters should be reversed in order.
'sir' back-to-front is 'ris'.
'i'+'ris'='IRIS'
'puts' acts as a link.
(Other definitions for iris that I've seen before include "bloomer?" , "pupil's governor" , "Flag - part of eye" , "plant with showy flowers" , "In the middle of which pupil is opening" .)