Rigid or unyielding (4-4)

I believe the answer is:

cast-iron

I believe this clue is a double definition.

'rigid' is the first definition.
(I've seen this before)

'unyielding' is the second definition.
(I've seen this before)

(Other definitions for cast-iron that I've seen before include "Extremely robust" , "Unbreakable" , "Unassailable" , "Rigid, unyielding" , "Very strong" .)

I've seen this clue in The Mirror.

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