Trio cease perversely making things obscure (9)
I believe the answer is:
esoterica
'things obscure' is the definition.
(obscure material or knowledge)
'trio cease perversely' is the wordplay.
'perversely' indicates an anagram.
'trio'+'cease'='triocease'
'triocease' anagrammed gives 'ESOTERICA'.
'making' acts as a link.
(Other definitions for esoterica that I've seen before include "Secret subjects" , "Arcane items" , "Mysteries" , "Highly-specialised subjects" , "Specialized publications" .)