Fell under the purview of the Athenaeum (5)
I believe the answer is:
heath
'fell' is the definition.
(thesaurus)
'of the athenaeum' is the wordplay.
'of' indicates a hidden word.
'HEATH' is hidden in the letters of 'the athenaeum'.
'under the purview' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for heath that I've seen before include "Common land; ex-PM" , "Sir Edward - , PM" , "Tract of open land" , "Shrubby area of open country" , "Moorland shrub" .)