Determined group (3)
I believe the answer is:
set
This clue is a double definition.
'determined' is the first definition.
(similar in meaning)
'group' is the second definition.
(both can mean a collection)
(Other definitions for set that I've seen before include "Group; unchanging" , "Hardened" , "Major unit of tennis match" , "Create a puzzle" , "'Put, place or fix in position (3)'" .)