Problem is masked in subtlety (8)
I believe the answer is:
nuisance
'problem' is the definition.
(thesaurus)
'masked in subtlety' is the wordplay.
'masked' becomes 'is' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'subtlety' becomes 'nuance' (synonyms).
'is' inserted inside 'nuance' is 'NUISANCE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for nuisance that I've seen before include "Bothersome annoying person" , "Source of annoyance or aggravation" , "Pest, source of annoyance" , "Source of irritation, person or thing" , "Irritant or botheration" .)