Cultured letter to a pope (6)
I believe the answer is:
urbane
'cultured' is the definition.
('urbane' can be a synonym of 'cultured')
'letter to a pope' is the wordplay.
'letter' becomes 'e'.
'to a' means one lot of letters go next to another (I've seen this in other clues).
'pope' becomes 'urban' (pope urban was a pope).
'e' after 'urban' is 'URBANE'.
(Other definitions for urbane that I've seen before include "Smooth" , "Sophisticated and elegant - sounds citified" , "Cosmopolitan" , "Suave and sophisticated" , "Refined and civilised" .)