Ideal example in a Post Office proposition about circular letter (10)
I believe the answer is:
apotheosis
'ideal example' is the definition.
'a post office proposition about circular letter' is the wordplay.
'post office' becomes 'po'.
'proposition' becomes 'thesis' (thesis is a kind of proposition**).
'about' is an insertion indicator.
'circular letter' becomes 'o'.
'thesis' enclosing 'o' is 'theosis'.
'a'+'po'+'theosis'='APOTHEOSIS'
'in' acts as a link.
(Other definitions for apotheosis that I've seen before include "Given maximum promotion" , "Highest point of development" , "Model" , "Deification" , "Elevation to divine status" .)