Shade of a nun seen round a religious establishment (6)
I believe the answer is:
nuance
'shade' is the definition.
(I know that shade can be written as nuance)
'a nun seen round a religious establishment' is the wordplay.
'seen round' means one lot of letters goes inside another.
'a religious establishment' becomes 'CE' (Church of England).
'a' put inside 'nun' is 'nuan'.
'nuan'+'ce'='NUANCE'
'of' acts as a link.
(Other definitions for nuance that I've seen before include "Subtle distinction or variation" , "Fine shade of meaning" , "Subtle difference of meaning" , "Subtle variation, in meaning say" , "Delicate shade of meaning" .)