Nuance (8)
'nuance' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for overtone that I've seen before include "Nuance" , "Innuendo perhaps" , "hidden meaning" , "Association" , "Subtle implication or connotation" .)
(Other definitions for subtlety that I've seen before include "Delicately understated quality" , "nice" , "'Nicety, nuance (8)'" , "Finesse" , "Ornamental device in sugar" .)