Firm and unchangeable (4-4)
I believe the answer is:
cast-iron
'firm and unchangeable' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cast-iron that I've seen before include "Extremely robust" , "Very strong" , "Firm and unchangeable" , "Rigid, unyielding" , "Unbreakable" .)