I recalled teacher is pupil’s neighbour (4)
I believe the answer is:
iris
'pupil's neighbour' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'i recalled teacher' is the wordplay.
'recalled' is a reversal indicator (to recall can mean to bring something back).
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'sir' backwards is 'ris'.
'i'+'ris'='IRIS'
'is' is the link.
Can you help me to learn more?
(Other definitions for iris that I've seen before include "Fleur-de-lys" , "bloomer?" , "Female" , "Fleur-de-lis" , "One controlling pupil' s reactions" .)