Subtle difference (6)
I believe the answer is:
nuance
'subtle difference' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nuance that I've seen before include "Delicate shade of meaning" , "A hint or shade" , "Subtlety, nicety" , "Delicate shade of difference" , "Slight difference in meaning etc" .)