Sister finally separating expert's head from body shows subtlety (6)
I believe the answer is:
nuance
'subtlety' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nuance that I've seen before include "A slight shade as of meaning" , "Subtle difference of meaning" , "Ace nun (anag)" , "Subtle variation, in meaning say" , "Subtle distinction or variation" .)