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" .)

I've seen this clue in The Independent.

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