Possible Clues:

• It can be hard to detect
• Subtle meaning is not found in an irritant
• Subtle difference of meaning
• The nuisance is gone and there's not much difference?
• Subtle shade of difference is not apparent in pest
• Just a shade as of meaning
• Has the nun got involved with the shady ace?
• Delicate shade of meaning
• In short, you can get in the shade here
• A slight shade or trace