Congeal or harden (3)
I believe the answer is:
set
This clue is a double definition.
'congeal' is the first definition.
(I know that congeal can be written as set)
'harden' is the second definition.
(set can mean to go hard or dry)
(Other definitions for set that I've seen before include "Determined or congealed" , "Cake < , plonk < , something served" , "Group; unchanging" , "plant" , "carriage" .)