Teacher's letter to the Inland Revenue (3)
I believe the answer is:
sir
'teacher's' is the definition.
(way of addressing a male teacher)
'letter to the inland revenue' is the wordplay.
'letter' becomes 's' ().
'to the inland revenue' becomes 'ir' (abbreviation for Inland Revenue).
's'+'ir'='SIR'
(Other definitions for sir that I've seen before include "Title given to baronet" , "Gentleman's form of address" , "... ___ John Major" , "Poet and novelist" , "Noble title of undesirable kind" .)