Harden; group (3)
I believe the answer is:
set
I believe this is a double definition.
'harden' is the first definition.
(set can mean to go hard or dry)
'group' is the second definition.
(both can mean a collection)
(Other definitions for set that I've seen before include "exclusive party" , "Cake < , plonk < , something served" , "Solidify like jelly" , "Prescribed" , "decline" .)