Determined to assume a certain hardness (3)
I believe the answer is:
set
'determined' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for set that I've seen before include "Group; unchanging" , "Mount (gem); scenery" , "Group; hard" , "number of games" , "Fix; lay" .)