One teacher turned up in place of pupil (4)

I believe the answer is:

iris

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'place of pupil' is the definition.
'iris' can be an answer for 'pupil' (thesaurus). I'm unsure of the 'place of' bit.

'one teacher turned up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'teacher' becomes 'sir' (way of addressing a male teacher).
'turned up' is a reversal indicator.
'sir' back-to-front is 'ris'.
'i'+'ris'='IRIS'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for iris that I've seen before include "Plant - part of the eye" , "Flag - flower - girl" , "Bright flower or eye part" , "Woman" , "Eye part or attractive flower" .)

I've seen this clue in The Sunday Times.

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