Refinement of sister touring a church (6)
I believe the answer is:
'refinement' is the definition.
(I know that refinement can be written as nuance)
'sister touring a church' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'touring' means one lot of letters goes inside another (tour can mean to go around).
'church' becomes 'CE' (Church of England).
'nun' enclosing 'a' is 'nuan'.
'of' acts as a link.
(Other definitions for nuance that I've seen before include "Subtle variation, in meaning say" , "Subtle shade of difference" , "Slight difference in meaning etc" , "Subtlety of meaning" , "Delicate shade of meaning" .)