Firm group (3)
I believe the answer is:
set
I believe this is a double definition.
'firm' is the first definition.
(both can mean fixed)
'group' is the second definition.
(both can mean a collection)
(Other definitions for set that I've seen before include "Scenery etc. for the stage" , "conventional" , "number of games" , "Firm, like jelly" , "Class" .)