Person penning international correspondence (6)
I believe the answer is:
parity
'correspondence' is the definition.
(I've seen this before)
'person penning international' is the wordplay.
'person' becomes 'party' (eg an individual in a court case).
'penning' indicates putting letters inside.
'international' becomes 'i' (abbreviation eg in IMF).
'party' going around 'i' is 'PARITY'.
(Other definitions for parity that I've seen before include "Egalitarian goal" , "Equality in status" , "Beginner" , "equal treatment" , "Equal condition" .)