Determined to become firm (3)
I believe the answer is:
set
I believe this is a double definition.
'determined' is the first definition.
(I've seen this before)
'become firm' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for set that I've seen before include "Congeal" , "Go below the horizon," , "Ready" , "Arrange; a party" , "Gel; start (a fire)" .)