Teacher, on reflection, is right (3)
I believe the answer is:
sir
'teacher' is the definition.
(I've seen this before)
'reflection is right' is the wordplay.
'reflection' is a reversal indicator.
'right' becomes 'r' (common abbreviation).
'is' in reverse letter order is 'si'.
'si'+'r'='SIR'
'on' is the link.
(Other definitions for sir that I've seen before include "Term of address for a gentleman" , "Gentleman's form of address" , "Polite or formal address to male" , "Title of 1 across" , "Knight's title" .)