Hardened group (3)
I believe the answer is:
set
This clue is a double definition.
'hardened' is the first definition.
(synonyms)
'group' is the second definition.
(both can mean a collection)
(Other definitions for set that I've seen before include "Arrange; a party" , "group usually found together" , "Where they're filming" , "declined" , "Harden; fixed; group" .)