Shame about a Republican's correspondence (6)
I believe the answer is:
parity
'correspondence' is the definition.
(I've seen this before)
'shame about a republican's' is the wordplay.
'shame' becomes 'pity' ('pity' can be a synonym of 'shame').
'about' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'a'+'r'='ar'
'pity' enclosing 'ar' is 'PARITY'.
(Other definitions for parity that I've seen before include "Egalitarian goal" , "Correspondence" , "Equal status or pay" , "Equality, equivalence" , "state of equality" .)