Subtle shading (6)
I believe the answer is:
nuance
'subtle shading' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nuance that I've seen before include "Cue Ann into slight shade" , "Delicate shade of difference" , "Slight difference in meaning etc" , "Subtlety, nicety" , "Subtle shade of meaning" .)