Firm collection (3)
I believe the answer is:
set
I believe this is a double definition.
'firm' is the first definition.
(both can mean fixed)
'collection' is the second definition.
(set is a kind of collection)
(Other definitions for set that I've seen before include "Put, coagulate" , "Put down tennis games" , "Arrange; a party" , "Ancient god" , "Become fixed or solid" .)