Annoyance is hidden by subtlety (8)
I believe the answer is:
nuisance
'annoyance' is the definition.
(synonyms)
'is hidden by subtlety' is the wordplay.
'hidden by' means one lot of letters goes inside another (the outside letters hide the inside ones).
'subtlety' becomes 'nuance' (synonyms).
'is' put inside 'nuance' is 'NUISANCE'.
(Other definitions for nuisance that I've seen before include "Thing causing annoyance" , "Source of irritation, person or thing" , "See 16" , "Pest, source of annoyance" , "Annoying or troublesome person or thing" .)