Subtle difference; nicety (6)
I believe the answer is:
nuance
This clue is a double definition.
'subtle difference' is the first definition.
(I've seen this in another clue)
'nicety' is the second definition.
(I know that nicety can be written as nuance)
(Other definitions for nuance that I've seen before include "A slight shade or trace" , "A slight shade as of meaning" , "Subtle variation, in meaning say" , "Subtle shade of meaning" , "Subtle difference of meaning" .)