Subtle difference in a sister taking one to church (6)
I believe the answer is:
nuance
'subtle difference' is the definition.
(I've seen this in another clue)
'sister taking one to church' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'taking' means one lot of letters goes inside another.
'one' becomes 'a' (a thing is one thing).
'to church' becomes 'CE' (abbreviation for Church of England).
'nun' enclosing 'a' is 'nuan'.
'nuan'+'ce'='NUANCE'
'in a' acts as a link.
(Other definitions for nuance that I've seen before include "Delicate shade of difference" , "Ace nun (anag)" , "Subtle shade of difference" , "Slight difference in meaning etc" , "Subtle distinction or variation" .)