Trace sister pinching a case of champagne (6)
I believe the answer is:
nuance
'trace' is the definition.
(I've seen this before)
'sister pinching a case of champagne' is the wordplay.
'sister' becomes 'nun' (I've seen this before).
'pinching' indicates putting letters inside (taking inside).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'champagne' with its middle taken out is 'ce'.
'nun' placed around 'a' is 'nuan'.
'nuan'+'ce'='NUANCE'
(Other definitions for nuance that I've seen before include "A slight shade as of meaning" , "Subtle shade of difference" , "Fine distinction" , "Fine shade of meaning" , "Subtlety of meaning" .)