Established group? (3)
I believe the answer is:
set
I believe this is a double definition.
'established' is the first definition.
(set can mean established or agreed upon)
'group?' is the second definition.
(both can mean a collection)
(Other definitions for set that I've seen before include "Prescribed" , "become firm" , "Scenery etc. for the stage" , "'Put, place or fix in position (3)'" , "Firm, like jelly" .)