Subtle distinction (6)
(Other definitions for nicety that I've seen before include "Subtle difference" , "Fine distinction" , "shade?" , "Judge to a . . . . . . - with precision" , "Small detail" .)
'subtle distinction' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nuance that I've seen before include "Subtle shade of meaning" , "Fine distinction" , "Subtle difference of meaning" , "Cue Ann into slight shade" , "Subtlety, nicety" .)